I have a PHP script with a MySQL backend that can create PureFTPD users, limit space, and limit bandwidth. The script will automatically disable accounts when the bandwidth limit is reached (not contact you, but I'm sure you could devise a script to check the status of the account from the DB). It doesn't have a billing component to it, either. If you're interested in the script, please PM me - I probably won't check this post again.
P.S. - This was a custom-programmed script for me, and not a commercially available script, however I am selling copies now.