Thread:Project:Support desk/Automatically Creating New User Accounts Once User Has Paid Subscription Fee/reply

Something like Extension:ConfirmAccount might help you, but you'd probably have to write your own extension to really do what you want. There's also extensions to work with other auth systems (like ldap for example - Extension:LDAP Authentication) This might allow you to integrate with an auth system that already has the integration into a payment system that you're looking for.