diff --git a/helpers.go b/helpers.go
index 1e4eac1..c692b1e 100644
--- a/helpers.go
+++ b/helpers.go
@@ -198,3 +198,15 @@ func typeNameAddrPtr(v any) string {
 	}
 	return rt.Name()
 }
+
+func concatBytes(s ...[]byte) []byte {
+	n := 0
+	for _, v := range s {
+		n += len(v)
+	}
+	res := make([]byte, 0, n)
+	for _, v := range s {
+		res = append(res, v...)
+	}
+	return res
+}
diff --git a/invoice.go b/invoice.go
index c13d805..2f7edbc 100644
--- a/invoice.go
+++ b/invoice.go
@@ -215,14 +215,14 @@ func (iv Invoice) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
 
 // XML returns the XML encoding of the Invoice
 func (iv Invoice) XML() ([]byte, error) {
-	return MarshalXML(iv)
+	return MarshalXMLWithHeader(iv)
 }
 
 // XMLIndent works like XML, but each XML element begins on a new
 // indented line that starts with prefix and is followed by one or more
 // copies of indent according to the nesting depth.
 func (iv Invoice) XMLIndent(prefix, indent string) ([]byte, error) {
-	return MarshalIndentXML(iv, prefix, indent)
+	return MarshalIndentXMLWithHeader(iv, prefix, indent)
 }
 
 // UnmarshalInvoice unmarshals an Invoice from XML data. Only use this method
diff --git a/xml_types.go b/xml_types.go
index d678a29..066ed0d 100644
--- a/xml_types.go
+++ b/xml_types.go
@@ -191,18 +191,39 @@ func NewIDNode(id string) *IDNode {
 
 // MarshalXML returns the XML encoding of v in Canonical XML form [XML-C14N].
 // This method must be used for marshaling objects from this library, instead
-// of encoding/xml.
+// of encoding/xml. This method does NOT include the XML header declaration.
 func MarshalXML(v any) ([]byte, error) {
 	return xml.Marshal(v)
 }
 
+// MarshalXMLWithHeader same as MarshalXML, but also add the XML header
+// declaration.
+func MarshalXMLWithHeader(v any) ([]byte, error) {
+	data, err := MarshalXML(v)
+	if err != nil {
+		return nil, err
+	}
+	return concatBytes([]byte(xml.Header), data), nil
+}
+
 // MarshalIndentXML works like MarshalXML, but each XML element begins on a new
 // indented line that starts with prefix and is followed by one or more
-// copies of indent according to the nesting depth.
+// copies of indent according to the nesting depth. This method does NOT
+// include the XML header declaration.
 func MarshalIndentXML(v any, prefix, indent string) ([]byte, error) {
 	return xml.MarshalIndent(v, prefix, indent)
 }
 
+// MarshalIndentXMLWithHeader same as MarshalIndentXML, but also add the XML
+// header declaration.
+func MarshalIndentXMLWithHeader(v any, prefix, indent string) ([]byte, error) {
+	data, err := MarshalIndentXML(v, prefix, indent)
+	if err != nil {
+		return nil, err
+	}
+	return concatBytes([]byte(xml.Header), data), nil
+}
+
 // Unmarshal parses the XML-encoded data and stores the result in
 // the value pointed to by v, which must be an arbitrary struct,
 // slice, or string. Well-formed data that does not fit into v is