[FEATURE] Create Checkout service to improve testing

[uncheckederror]

What is the problem that this feature solves?
Because the invoice generation process is split between the Ops and Mvc apps they often get out of sync and are difficult to test.

Describe the solution you'd like
I'd like a single service that takes a cart and then produces invoices based on that cart.

How will this improve revenue or reduce our costs?
We often have to fix bugs when the invoice generation process breaks.