First round using whmcs went very smoothly. Each portal user had all backup accounts itemized by backup account name in a single invoice. whmcs supports credit card, so this could be a billing solution if chosen.

There is now officially enough money invested into this code that I'm stepping back and evaluating implementing a real licensing model where tracking and enforcement of hosted code is automated. Coupled with the need to go through one more billing cycle to ensure this release is quality, I'm moving the release date to mid October.