You could write your own script using the WHM API so that when you have confirmed payment (most payment processors will send a POST request to your website) you trigger the API to setup the account.
Little tip: make sure that when creating an account automatically that a) the username is valid (i.e not too long/illegal characters) and b) the username doesn't already exist. To check that the account has been set up, you can do a list of all accounts and then check the list for the username you tried to register.
Sounds simple in theory
