[Kaelon]

Your question is far more appropriate for PayPal personnel, given that it largely has to do with PayPal's recurring scheme. That said, PayPal has recurring subscriptions that you can define on the fly (e.g., $55 monthly, recurring until either the subscriber or the merchant). However, PayPal does not support partial invoicing - that is, you cannot send them a bill, and have the customers pay a portion of that bill. Furthermore, PayPal, 2CheckOut, and other providers, take both a flat amount as well as a percentage of the sale (usually between 3.9% and 5.5%, depending upon the provider and the amount on the sale) on commission with each transaction.

I hope this helps. Honestly, you'll want to take a look at PayPal and judge for yourself the flexibility. Though the latest version of vBulletin (3.0.0) offers an unprecedented level of integration with user accounts and subscription-level access, we are a far way away from full control using PayPal and other lower-end merchant solutions. It would probably be easier getting a merchant credit card account, and coding your own software to enable the highly specialized functionality that you seek.

That said, for non-complicated billing solutions - the types that most content-based websites do, like mine, using PayPal is pretty simple.