Add New Membership Subscription to Personals Ad Website

The script can be found here: [login to view URL] What we want to add is a feature that forces a user to subscribe and buy credits in order to respond to ads on the site. After successful payment (for now paypal and moneybookers, but later want to add credit cards), the user should be taken back to the ad and be allowed to make contact. We want the ability to add numerous payment types and then once the user has bought credits, they will be deducted from their account every time they reply to an ad.