Allow Users to Provide Payment and Subscribe to My Site

Greetings - I have a free site that will soon allow users to become subscibers and then access premium pages and features within the site. I currently have two ASP.Net "roles" configured: one "free" and the other "paid". I need someone who can help me tie in my site to a third party site where a user submits payment, then their role is automatically changed from "free" to "paid". I plan on using E-Junkie.com. I need someone who has experience doing this sort of thing. Thanks for reading.