Plesk 12 offers a turn-key solution for users looking to resell hosting. Billing is integrated into the administrator section of the panel where you can set packages and pricing. When your customers log into their panel the first screen will show them if they have any outstanding invoices due and allows them to pay.
Parallels Plesk Panel 12.5 Demo
Parallels offers a demo of the admin panel for Plesk 12.5:
Demo Parallels Plesk Panel as an Administrator
Documentation for Plesk is available and can be found here: