I have created the library using Rust. For details on how to install Rust, visit the Rust website. The tests can be run using cargo test
from the project directory. The exposed API would likely consist of Parcel::new and Order::new, which create new parcels and orders respectively, and Order::produce_invoice which
produces the itemised invoice for the order.
In the two hours, I successfully created a library that performs the tasks set out in parts 1-4. The code is modular and easily extensible, with good test coverage.
I ran out of time to implement part 5. I began laying the framework for this by adding a sort, but I didn't have time to do any more, though I do have ideas on what I could've done next. For example, turn the list into an iterator and consume parcels while adding discounts to the invoice as necessary - though I was still mulling over some of the finer details on how to ensure the discount was optimal. At worst, I figured that I could always use recursion to brute-force an optimal solution.
I also would have liked to perform further testing on edge cases and perhaps handled erroneous or float inputs.