[support] How to make payment part of a user creation.

Dear All, I am trying to set up a Drupal site where online services will be sold. A user can only exist if she/he completed a payment (i.e. we will not have free users). From my understanding this means the payment itself is a prerequisite to create a user; it is part of the user creation itself. I have looked at UBERCART. I have been able to install/configure and integrate it with my payment gateway. Now I have a cart and a user can "purchase". All that works fine... But I am kinda lost because what I want is to integrate this payment as part of the user creation. Could anyone let me know where should I start looking at? Thank you so much! unai