Following the online payments project launch, this was a logical next step as some customers would still be ordering first and not paying until later.
I did a similar thing when I worked for King Street Computers in Australia, so I already had a clear idea of how it would work. Replacing an antiquated Word invoicing setup, I developed the ability for customers to download invoice PDFs and pay them through the My Account screens. This then fed through to the back-end screens and Finance could see which orders were paid or unpaid. I also developed a simple REST API so the accounting system (Opera) could mark invoices as paid.
Project: Online invoicing
Launch: March 2012
Role: Lead Developer