How about integrating clickbank as a payment provider into Xenforo instead of just paypal? They have a great recurring billing feature and will follow-up on bounced cards and expiration dates. Also they have a built in affiliate program and handle all payments to your affiliates (something that paypal doesn't offer). How difficult would it be to add them as a payment provider? Full integration would be nice so that if people stop paying, they're access to Xenforo is cut off.