diff --git a/ShippingRates.Tests/Helpers/DHLServicesValidatorTests.cs b/ShippingRates.Tests/Helpers/DHLServicesValidatorTests.cs
index 5f8ecef..1bb8949 100644
--- a/ShippingRates.Tests/Helpers/DHLServicesValidatorTests.cs
+++ b/ShippingRates.Tests/Helpers/DHLServicesValidatorTests.cs
@@ -1,7 +1,8 @@
 using NUnit.Framework;
+using ShippingRates.Helpers;
 using System.Linq;
 
-namespace ShippingRates.Helpers.Tests
+namespace ShippingRates.Tests.Helpers
 {
     [TestFixture()]
     public class DHLServicesValidatorTests
diff --git a/ShippingRates.Tests/Helpers/Extensions/AddressExtensionsTests.cs b/ShippingRates.Tests/Helpers/Extensions/AddressExtensionsTests.cs
index 0f6fa6a..b2dcf2f 100644
--- a/ShippingRates.Tests/Helpers/Extensions/AddressExtensionsTests.cs
+++ b/ShippingRates.Tests/Helpers/Extensions/AddressExtensionsTests.cs
@@ -1,6 +1,7 @@
 using NUnit.Framework;
+using ShippingRates.Helpers.Extensions;
 
-namespace ShippingRates.Helpers.Extensions.Tests
+namespace ShippingRates.Tests.Helpers.Extensions
 {
     [TestFixture()]
     public class AddressExtensionsTests
diff --git a/ShippingRates.Tests/Models/ShipmentTests.cs b/ShippingRates.Tests/Models/ShipmentTests.cs
index 6770fee..d7e5f62 100644
--- a/ShippingRates.Tests/Models/ShipmentTests.cs
+++ b/ShippingRates.Tests/Models/ShipmentTests.cs
@@ -1,12 +1,7 @@
 using NUnit.Framework;
-using ShippingRates;
-using System;
 using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 
-namespace ShippingRates.Tests
+namespace ShippingRates.Tests.Models
 {
     [TestFixture()]
     public class ShipmentTests
@@ -26,9 +21,12 @@ public void ShipmentTest_HasDocumentsOnly()
             var shipmentWithSomeDocs = new Shipment(from, to, new List<Package>() { package1, docsPackage1, package2 });
             var shipmentAllDocs = new Shipment(from, to, new List<Package>() { docsPackage1, docsPackage2 });
 
-            Assert.IsFalse(shipmentNoDocs.HasDocumentsOnly);
-            Assert.IsFalse(shipmentWithSomeDocs.HasDocumentsOnly);
-            Assert.IsTrue(shipmentAllDocs.HasDocumentsOnly);
+            Assert.Multiple(() =>
+            {
+                Assert.That(shipmentNoDocs.HasDocumentsOnly, Is.False);
+                Assert.That(shipmentWithSomeDocs.HasDocumentsOnly, Is.False);
+                Assert.That(shipmentAllDocs.HasDocumentsOnly, Is.True);
+            });
         }
     }
 }
diff --git a/ShippingRates.Tests/ShippingProviders/FedExShipRates.cs b/ShippingRates.Tests/ShippingProviders/FedExShipRates.cs
index 3121c3a..88fdf53 100644
--- a/ShippingRates.Tests/ShippingProviders/FedExShipRates.cs
+++ b/ShippingRates.Tests/ShippingProviders/FedExShipRates.cs
@@ -58,12 +58,15 @@ public void FedExReturnsRates()
             var r = _rateManager.GetRates(from, to, package);
             var fedExRates = r.Rates.ToList();
 
-            Assert.NotNull(r);
-            Assert.True(fedExRates.Any());
+            Assert.Multiple(() =>
+            {
+                Assert.That(r, Is.Not.Null);
+                Assert.That(fedExRates, Is.Not.Empty);
+            });
 
             foreach (var rate in fedExRates)
             {
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
             }
         }
 
@@ -74,18 +77,23 @@ public void FedExReturnsErrors()
             var to = new Address("", "", "30404", "US");
             var package = new Package(7, 7, 7, 6, 1);
 
-            var r = _rateManager.GetRates(from, to, package);
-            var fedExRates = r.Rates.ToList();
+            var rates = _rateManager.GetRates(from, to, package);
 
-            Assert.NotNull(r);
-            Assert.False(fedExRates.Any());
-            Assert.AreEqual(r.Errors.Count, 1);
+            Assert.Multiple(() =>
+            {
+                Assert.That(rates, Is.Not.Null);
+                Assert.That(rates.Rates, Is.Empty);
+            });
+            Assert.That(rates.Errors, Has.Count.EqualTo(1));
 
-            var error = r.Errors.FirstOrDefault();
-            Assert.NotNull(error);
-            Assert.AreEqual(error.Number, "521");
-            Assert.NotNull(error.Description);
-            Assert.AreEqual(error.Description[..42], "Destination postal code missing or invalid");
+            var error = rates.Errors.FirstOrDefault();
+            Assert.That(error, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(error.Number, Is.EqualTo("521"));
+                Assert.That(error.Description, Is.Not.Null);
+            });
+            Assert.That(error.Description[..42], Is.EqualTo("Destination postal code missing or invalid"));
         }
 
 
@@ -149,28 +157,40 @@ public void FedExFreight()
 
             var rates = _rateManager.GetRates(from, to, package);
 
-            Assert.True(rates.Rates.Any());
-            Assert.True(!rates.Rates.Any(r => !r.Name.Contains("Freight")));
+            Assert.Multiple(() =>
+            {
+                Assert.That(rates.Rates, Is.Not.Empty);
+                Assert.That(rates.Rates.All(r => r.Name.Contains("Freight")), Is.True);
+            });
         }
 
         private static void AssertRatesAreNotEqual(Shipment r1, Shipment r2, string methodCode = null)
         {
-            Assert.NotNull(r1?.Rates);
-            Assert.NotNull(r2?.Rates);
-            Assert.True(r1.Rates.Any());
-            Assert.True(r2.Rates.Any());
+            Assert.Multiple(() =>
+            {
+                Assert.That(r1?.Rates, Is.Not.Null);
+                Assert.That(r2?.Rates, Is.Not.Null);
+            });
+            Assert.Multiple(() =>
+            {
+                Assert.That(r1.Rates, Is.Not.Empty);
+                Assert.That(r2.Rates, Is.Not.Empty);
+            });
 
             var commonCode = methodCode ?? r1.Rates
                 .Select(r => r.ProviderCode)
                 .Where(c => r2.Rates.Select(r => r.ProviderCode).Contains(c))
                 .FirstOrDefault();
-            Assert.NotNull(commonCode);
+            Assert.That(commonCode, Is.Not.Null);
 
             var rate1 = r1.Rates.FirstOrDefault(r => r.ProviderCode == commonCode);
             var rate2 = r2.Rates.FirstOrDefault(r => r.ProviderCode == commonCode);
-            Assert.NotNull(rate1);
-            Assert.NotNull(rate2);
-            Assert.AreNotEqual(rate1.TotalCharges, rate2.TotalCharges);
+            Assert.Multiple(() =>
+            {
+                Assert.That(rate1, Is.Not.Null);
+                Assert.That(rate2, Is.Not.Null);
+            });
+            Assert.That(rate2.TotalCharges, Is.Not.EqualTo(rate1.TotalCharges));
         }
 
         [Test]
@@ -188,11 +208,10 @@ public async Task FedExSaturdayDelivery()
                 ShippingDate = nextFriday,
                 SaturdayDelivery = true
             });
-            var fedExRates = r.Rates.ToList();
 
-            Assert.NotNull(r);
-            Assert.True(fedExRates.Any());
-            Assert.True(fedExRates.Any(r => r.Options.SaturdayDelivery));
+            Assert.That(r, Is.Not.Null);
+            Assert.That(r.Rates, Is.Not.Empty);
+            Assert.That(r.Rates.Any(r => r.Options.SaturdayDelivery), Is.True);
         }
 
         [Test]
@@ -203,11 +222,10 @@ public async Task FedExCurrency()
             var package = new Package(1, 1, 1, 5, 1);
 
             var r = await _rateManager.GetRatesAsync(from, to, package);
-            var fedExRates = r.Rates.ToList();
 
-            Assert.NotNull(r);
-            Assert.True(fedExRates.Any());
-            Assert.False(fedExRates.Any(r => r.TotalCharges > 1000));
+            Assert.That(r, Is.Not.Null);
+            Assert.That(r.Rates, Is.Not.Empty);
+            Assert.That(r.Rates.Any(r => r.TotalCharges > 1000), Is.False);
         }
 
         [Test]
@@ -221,11 +239,10 @@ public async Task FedExPreferredCurrency()
             {
                 PreferredCurrencyCode = "USD"
             });
-            var fedExRates = r.Rates.ToList();
 
-            Assert.NotNull(r);
-            Assert.True(fedExRates.Any());
-            Assert.False(fedExRates.Any(r => r.CurrencyCode != "USD"));
+            Assert.That(r, Is.Not.Null);
+            Assert.That(r.Rates, Is.Not.Empty);
+            Assert.That(r.Rates.Any(r => r.CurrencyCode != "USD"), Is.False);
 
             var rEuro = await _rateManager.GetRatesAsync(from, to, package, new ShipmentOptions()
             {
@@ -233,9 +250,9 @@ public async Task FedExPreferredCurrency()
             });
             var fedExEuroRates = rEuro.Rates.ToList();
 
-            Assert.NotNull(rEuro);
-            Assert.True(fedExEuroRates.Any());
-            Assert.False(fedExEuroRates.Any(r => r.CurrencyCode != "EUR"));
+            Assert.That(rEuro, Is.Not.Null);
+            Assert.That(rEuro.Rates, Is.Not.Empty);
+            Assert.That(rEuro.Rates.Any(r => r.CurrencyCode != "EUR"), Is.False);
         }
 
         /*
@@ -293,8 +310,8 @@ public void CanGetFedExServiceCodes()
         {
             var serviceCodes = _provider.GetServiceCodes();
 
-            Assert.NotNull(serviceCodes);
-            Assert.IsNotEmpty(serviceCodes);
+            Assert.That(serviceCodes, Is.Not.Null);
+            Assert.That(serviceCodes, Is.Not.Empty);
         }
     }
 }
diff --git a/ShippingRates.Tests/ShippingProviders/UPSRates.cs b/ShippingRates.Tests/ShippingProviders/UPSRates.cs
index 70e11ae..dd115a5 100644
--- a/ShippingRates.Tests/ShippingProviders/UPSRates.cs
+++ b/ShippingRates.Tests/ShippingProviders/UPSRates.cs
@@ -11,12 +11,12 @@ namespace ShippingRates.Tests.ShippingProviders
     [TestFixture]
     public class UPSRates
     {
-        private Address InternationalAddress2;
-        private Package Package2;
         private readonly Address DomesticAddress1;
         private readonly Address DomesticAddress2;
         private readonly Address InternationalAddress1;
+        private readonly Address InternationalAddress2;
         private readonly Package Package1;
+        private readonly Package Package2;
         private readonly Package Package1SignatureRequired;
         private readonly string UPSAccountNumber;
         private readonly string UPSClientId;
@@ -50,14 +50,17 @@ public void UPS_Domestic_Returns_Rates_When_Using_International_Addresses_For_Si
 
             Debug.WriteLine($"Rates returned: {(response.Rates.Any() ? response.Rates.Count.ToString() : "0")}");
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -73,14 +76,17 @@ public void UPS_Returns_Multiple_Rates_When_Using_Valid_Addresses_For_All_Servic
 
             Debug.WriteLine($"Rates returned: {(response.Rates.Any() ? response.Rates.Count.ToString() : "0")}");
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -96,14 +102,17 @@ public void UPS_Returns_Multiple_Rates_When_Using_Valid_Addresses_For_All_Servic
 
             Debug.WriteLine($"Rates returned: {(response.Rates.Any() ? response.Rates.Count.ToString() : "0")}");
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -170,14 +179,17 @@ public void UPS_Returns_Rates_When_Using_International_Destination_Addresses_For
 
             Debug.WriteLine($"Rates returned: {(response.Rates.Any() ? response.Rates.Count.ToString() : "0")}");
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -196,13 +208,16 @@ public void UPS_Returns_Single_Rate_When_Using_Domestic_Addresses_For_Single_Ser
 
             Debug.WriteLine($"Rates returned: {(response.Rates.Any() ? response.Rates.Count.ToString() : "0")}");
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
-            Assert.AreEqual(response.Rates.Count, 1);
-            Assert.Greater(response.Rates.First().TotalCharges, 0);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
+            Assert.That(response.Rates, Has.Count.EqualTo(1));
+            Assert.That(response.Rates.First().TotalCharges, Is.GreaterThan(0));
             // Expect something around $25, not $2500
-            Assert.Less(response.Rates.First().TotalCharges, 100);
+            Assert.That(response.Rates.First().TotalCharges, Is.LessThan(100));
 
             Debug.WriteLine(response.Rates.First().Name + ": " + response.Rates.First().TotalCharges);
         }
@@ -212,8 +227,8 @@ public void CanGetUpsServiceCodes()
         {
             var serviceCodes = UPSProvider.GetServiceCodes();
 
-            Assert.NotNull(serviceCodes);
-            Assert.IsNotEmpty(serviceCodes);
+            Assert.That(serviceCodes, Is.Not.Null);
+            Assert.That(serviceCodes, Is.Not.Empty);
         }
 
         [Test]
@@ -227,19 +242,25 @@ public void Can_Get_Different_Rates_For_Signature_Required_Lookup()
 
             Debug.WriteLine(string.Format("Rates returned: {0}", nonSignatureResponse.Rates.Any() ? nonSignatureResponse.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(nonSignatureResponse);
-            Assert.IsNotEmpty(nonSignatureResponse.Rates);
-            Assert.IsEmpty(nonSignatureResponse.Errors);
-            Assert.AreEqual(nonSignatureResponse.Rates.Count, 1);
-            Assert.True(nonSignatureResponse.Rates.First().TotalCharges > 0);
+            Assert.That(nonSignatureResponse, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(nonSignatureResponse.Rates, Is.Not.Empty);
+                Assert.That(nonSignatureResponse.Errors, Is.Empty);
+            });
+            Assert.That(nonSignatureResponse.Rates, Has.Count.EqualTo(1));
+            Assert.That(nonSignatureResponse.Rates.First().TotalCharges, Is.GreaterThan(0));
 
             Debug.WriteLine(string.Format("Rates returned: {0}", signatureResponse.Rates.Any() ? signatureResponse.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(signatureResponse);
-            Assert.IsNotEmpty(signatureResponse.Rates);
-            Assert.IsEmpty(signatureResponse.Errors);
-            Assert.AreEqual(signatureResponse.Rates.Count, 1);
-            Assert.True(signatureResponse.Rates.First().TotalCharges > 0);
+            Assert.That(signatureResponse, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(signatureResponse.Rates, Is.Not.Empty);
+                Assert.That(signatureResponse.Errors, Is.Empty);
+            });
+            Assert.That(signatureResponse.Rates, Has.Count.EqualTo(1));
+            Assert.That(signatureResponse.Rates.First().TotalCharges, Is.GreaterThan(0));
 
             // Now compare prices
             foreach (var signatureRate in signatureResponse.Rates)
@@ -250,7 +271,7 @@ public void Can_Get_Different_Rates_For_Signature_Required_Lookup()
                 {
                     var signatureTotalCharges = signatureRate.TotalCharges;
                     var nonSignatureTotalCharges = nonSignatureRate.TotalCharges;
-                    Assert.AreNotEqual(signatureTotalCharges, nonSignatureTotalCharges);
+                    Assert.That(nonSignatureTotalCharges, Is.Not.EqualTo(signatureTotalCharges));
                 }
             }
         }
@@ -275,9 +296,12 @@ public async Task UPSSaturdayDelivery()
             });
             var fedExRates = r.Rates.ToList();
 
-            Assert.NotNull(r);
-            Assert.True(fedExRates.Any());
-            Assert.True(fedExRates.Any(r => r.Options.SaturdayDelivery));
+            Assert.Multiple(() =>
+            {
+                Assert.That(r, Is.Not.Null);
+                Assert.That(fedExRates.Any(), Is.True);
+                Assert.That(fedExRates.Any(r => r.Options.SaturdayDelivery), Is.True);
+            });
         }
 
         //// Euro rates for shipping in Europe
diff --git a/ShippingRates.Tests/ShippingProviders/USPSInternationalRates.cs b/ShippingRates.Tests/ShippingProviders/USPSInternationalRates.cs
index e1d956d..a4699a8 100644
--- a/ShippingRates.Tests/ShippingProviders/USPSInternationalRates.cs
+++ b/ShippingRates.Tests/ShippingProviders/USPSInternationalRates.cs
@@ -61,14 +61,17 @@ public void USPS_Intl_Returns_Multiple_Rates_When_Using_Multiple_Packages_For_Al
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -85,14 +88,17 @@ public void USPS_Intl_Returns_Multiple_Rates_When_Using_Valid_Addresses_For_All_
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -109,8 +115,8 @@ public void USPS_Intl_Returns_No_Rates_When_Using_Invalid_Addresses_For_All_Serv
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsEmpty(response.Rates);
+            Assert.That(response, Is.Not.Null);
+            Assert.That(response.Rates, Is.Empty);
         }
 
         [Test]
@@ -126,8 +132,8 @@ public void USPS_Intl_Returns_No_Rates_When_Using_Invalid_Addresses_For_Single_S
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsEmpty(response.Rates);
+            Assert.That(response, Is.Not.Null);
+            Assert.That(response.Rates, Is.Empty);
         }
 
         [Test]
@@ -141,11 +147,13 @@ public void USPS_Intl_Returns_Single_Rate_When_Using_Valid_Addresses_For_Single_
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
-            Assert.AreEqual(response.Rates.Count, 1);
-            Assert.True(response.Rates.First().TotalCharges > 0);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Errors, Is.Empty);
+                Assert.That(response.Rates, Has.Count.EqualTo(1));
+            });
+            Assert.That(response.Rates.First().TotalCharges, Is.GreaterThan(0));
 
             Debug.WriteLine(response.Rates.First().Name + ": " + response.Rates.First().TotalCharges);
         }
@@ -158,7 +166,7 @@ public void USPS_Intl_Returns_First_Class_Mail_Rates_For_FirstClassLetterWithNoV
             rateManager.AddProvider(new USPSInternationalProvider(GetConfiguration(), httpClient));
 
             var response = rateManager.GetRates(_domesticAddress1, _internationalAddress2, _firstClassLetterWithNoValue);
-            Assert.True(response.Rates.Any(IsFirstClassMailRate));
+            Assert.That(response.Rates.Any(IsFirstClassMailRate), Is.True);
         }
 
         [Test]
@@ -169,7 +177,7 @@ public void USPS_Intl_Returns_No_First_Class_Mail_Rates_For_FirstClassLetterWith
             rateManager.AddProvider(new USPSInternationalProvider(GetConfiguration(), httpClient));
 
             var response = rateManager.GetRates(_domesticAddress1, _internationalAddress2, _firstClassLetterWithValue);
-            Assert.False(response.Rates.Any(IsFirstClassMailRate));
+            Assert.That(response.Rates.Any(IsFirstClassMailRate), Is.False);
         }
 
         [Test]
@@ -182,7 +190,7 @@ public void USPS_Intl_Returns_No_First_Class_Mail_Rates_For_Package()
             var package = new Package(7m, 5m, 0.1m, FirstClassLetterMaxWeight, 0);
 
             var response = rateManager.GetRates(_domesticAddress1, _internationalAddress2, package);
-            Assert.False(response.Rates.Any(IsFirstClassMailRate));
+            Assert.That(response.Rates.Any(IsFirstClassMailRate), Is.False);
         }
 
         [Test]
@@ -191,8 +199,8 @@ public void CanGetUspsInternationalServiceCodes()
             var provider = new USPSInternationalProvider(GetConfiguration());
             var serviceCodes = provider.GetServiceCodes();
 
-            Assert.NotNull(serviceCodes);
-            Assert.IsNotEmpty(serviceCodes);
+            Assert.That(serviceCodes, Is.Not.Null);
+            Assert.That(serviceCodes, Is.Not.Empty);
         }
 
         [Test]
@@ -205,9 +213,12 @@ public async Task USPSCurrency()
             var response = await rateManager.GetRatesAsync(_domesticAddress1, _internationalAddress2, _package1);
             var rates = response.Rates;
 
-            Assert.NotNull(response);
-            Assert.True(rates.Any());
-            Assert.False(rates.Any(r => r.CurrencyCode != "USD"));
+            Assert.Multiple(() =>
+            {
+                Assert.That(response, Is.Not.Null);
+                Assert.That(rates.Any(), Is.True);
+                Assert.That(rates.Any(r => r.CurrencyCode != "USD"), Is.False);
+            });
         }
 
         private bool IsFirstClassMailRate(Rate rate)
diff --git a/ShippingRates.Tests/ShippingProviders/USPSProviderTests.cs b/ShippingRates.Tests/ShippingProviders/USPSProviderTests.cs
index b4c4098..a2186cc 100644
--- a/ShippingRates.Tests/ShippingProviders/USPSProviderTests.cs
+++ b/ShippingRates.Tests/ShippingProviders/USPSProviderTests.cs
@@ -51,15 +51,18 @@ public void USPS_Domestic_Returns_Multiple_Rates_When_Using_Valid_Addresses_For_
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
-            Assert.IsEmpty(response.InternalErrors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+                Assert.That(response.InternalErrors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -76,14 +79,17 @@ public void USPS_Domestic_Returns_Multiple_Rates_When_Using_Valid_Addresses_For_
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -100,9 +106,12 @@ public void USPS_Domestic_Returns_No_Rates_When_Using_Invalid_Addresses_For_All_
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
         }
 
         [Test]
@@ -116,9 +125,12 @@ public void USPS_Domestic_Returns_No_Rates_When_Using_Invalid_Addresses_For_Sing
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
         }
 
         [Test]
@@ -132,11 +144,14 @@ public void USPS_Domestic_Returns_Single_Rate_When_Using_Valid_Addresses_For_Sin
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
-            Assert.IsEmpty(response.Errors);
-            Assert.AreEqual(response.Rates.Count, 1);
-            Assert.True(response.Rates.First().TotalCharges > 0);
+            Assert.That(response, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Rates, Is.Not.Empty);
+                Assert.That(response.Errors, Is.Empty);
+            });
+            Assert.That(response.Rates, Has.Count.EqualTo(1));
+            Assert.That(response.Rates.First().TotalCharges, Is.GreaterThan(0));
 
             Debug.WriteLine(response.Rates.First().Name + ": " + response.Rates.First().TotalCharges);
         }
@@ -147,8 +162,8 @@ public void CanGetUspsServiceCodes()
             var provider = new USPSProvider(GetConfiguration());
             var serviceCodes = provider.GetServiceCodes();
 
-            Assert.NotNull(serviceCodes);
-            Assert.IsNotEmpty(serviceCodes);
+            Assert.That(serviceCodes, Is.Not.Null);
+            Assert.That(serviceCodes, Is.Not.Empty);
         }
 
         [Test]
@@ -229,10 +244,13 @@ public void USPSDiscountedRates()
 
         private static void AssertIsValidNonEmptyResponse(Shipment shipment)
         {
-            Assert.NotNull(shipment);
-            Assert.IsNotEmpty(shipment.Rates);
-            Assert.IsEmpty(shipment.Errors);
-            Assert.True(shipment.Rates.First().TotalCharges > 0);
+            Assert.That(shipment, Is.Not.Null);
+            Assert.Multiple(() =>
+            {
+                Assert.That(shipment.Rates, Is.Not.Empty);
+                Assert.That(shipment.Errors, Is.Empty);
+            });
+            Assert.That(shipment.Rates.First().TotalCharges, Is.GreaterThan(0));
         }
 
         private static void AssertRatesAreDifferent(List<Rate> ratesA, List<Rate> ratesB)
@@ -249,7 +267,7 @@ private static void AssertRatesAreDifferent(List<Rate> ratesA, List<Rate> ratesB
                     break;
             }
 
-            Assert.IsTrue(hasDifference);
+            Assert.That(hasDifference, Is.True);
         }
 
         [Test]
@@ -272,8 +290,8 @@ public async Task USPS_Domestic_Saturday_Delivery()
                 SaturdayDelivery = true
             });
 
-            Assert.NotNull(response);
-            Assert.IsNotEmpty(response.Rates);
+            Assert.That(response, Is.Not.Null);
+            Assert.That(response.Rates, Is.Not.Empty);
 
             // Sometimes only Priority Mail Express 2-Day works and we have to try it on Thursday
             if (!response.Rates.Any(r => r.Options.SaturdayDelivery))
@@ -284,19 +302,22 @@ public async Task USPS_Domestic_Saturday_Delivery()
                     SaturdayDelivery = true
                 });
 
-                Assert.NotNull(response);
-                Assert.IsNotEmpty(response.Rates);
+                Assert.That(response, Is.Not.Null);
+                Assert.That(response.Rates, Is.Not.Empty);
             }
 
             Debug.WriteLine(string.Format("Rates returned: {0}", response.Rates.Any() ? response.Rates.Count.ToString() : "0"));
 
-            Assert.IsEmpty(response.Errors);
-            Assert.True(response.Rates.Any(r => r.Options.SaturdayDelivery));
+            Assert.Multiple(() =>
+            {
+                Assert.That(response.Errors, Is.Empty);
+                Assert.That(response.Rates, Has.Some.Matches<Rate>(r => r.Options.SaturdayDelivery));
+            });
 
             foreach (var rate in response.Rates)
             {
-                Assert.NotNull(rate);
-                Assert.True(rate.TotalCharges > 0);
+                Assert.That(rate, Is.Not.Null);
+                Assert.That(rate.TotalCharges, Is.GreaterThan(0));
 
                 Debug.WriteLine(rate.Name + ": " + rate.TotalCharges);
             }
@@ -317,7 +338,7 @@ public async Task USPS_ThreeAndAHalfOunceLetter_Qualifies_For_First_Class_Mail_L
 
             var response = await rateManager.GetRatesAsync(origin, destination, firstClassLetter);
 
-            Assert.True(response.Rates.Any(r =>
+            Assert.That(response.Rates, Has.Some.Matches<Rate>(r =>
                 r.ProviderCode is "First-Class Mail Stamped Letter" or "First-Class Mail Metered Letter"));
         }
     }
diff --git a/ShippingRates.Tests/ShippingRates.Tests.csproj b/ShippingRates.Tests/ShippingRates.Tests.csproj
index 8ab8fc2..02f4328 100644
--- a/ShippingRates.Tests/ShippingRates.Tests.csproj
+++ b/ShippingRates.Tests/ShippingRates.Tests.csproj
@@ -10,14 +10,10 @@
     <PackageReference Include="Microsoft.Extensions.Configuration.Binder" Version="8.0.2" />
     <PackageReference Include="Microsoft.Extensions.Configuration.EnvironmentVariables" Version="8.0.0" />
     <PackageReference Include="Microsoft.Extensions.Configuration.Json" Version="8.0.0" />
-    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.0" />
-    <PackageReference Include="MSTest.TestAdapter" Version="3.5.2" />
-    <PackageReference Include="MSTest.TestFramework" Version="3.5.2" />
-    <PackageReference Include="coverlet.collector" Version="3.1.2">
-      <PrivateAssets>all</PrivateAssets>
-      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
-    </PackageReference>
-    <PackageReference Include="NUnit" Version="3.14.0" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.11.1" />
+    <PackageReference Include="MSTest.TestAdapter" Version="3.6.0" />
+    <PackageReference Include="MSTest.TestFramework" Version="3.6.0" />
+    <PackageReference Include="NUnit" Version="4.2.2" />
     <PackageReference Include="NUnit.Analyzers" Version="4.3.0">
       <PrivateAssets>all</PrivateAssets>
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>