Most billing systems will allow you to automatically create accounts when payment is recieved. At the same time, this can cause a problem in that if a fraudster/spammer signs up, he can signup and send thousands of emails if it's hours before you get a chance to check his account. You need to balance whether the user waiting a few hours for you to check them out is that big a problem vs the potential problems you could have.

I do believe, at least in the past, Clientexec could also be setup to do things automatically using a user submitted cron job - not sure if that still works anymore.