Payflow Pro (by PayPal) Recurring Billing. I'm sure there are much, much better APIs out there but most of my comments would apply to any of them. EDIT: tl;dr

Payflow Pro (by PayPal) Recurring Billing. I'm sure there are much, much better APIs out there but most of my comments would apply to any of them. EDIT: tl;dr for my above comment: Third-party solutions are basically a replacement of a cron job that figures out when to bill someone and how much. It's easier to write that cron job than it is to keep your billing data synchronized across two systems.