You could save the registration information to the database using Data Bridge, then use eCart to add the registration to the cart and create the checkout pages. When the registration is first saved, have a column for paid and set it to false by default
after checkout is complete, you would need to create an update to update the paid status to true.
if you need help setting this up, we can help through a premiere support appointment:
http://www.webassist.com/premier_request.php