Registration and payment would be separate from each other.
rather than have payment on the same page as the registration, you would set up the registration to allow the user basic access to the site. once registered, the user can purchase the more advanced membership.
After the checkout has completed, update the users access level to allow access to the rest of the site.
The Archived Documentation section of the security assist support page has a User Level authentication tutorial, it was written for an earlier version of security assist, but the concept is the same.
you would need to use eCart for adding the membership to the cart and checking out.