Audible.com/e-commerce/credit system Like Functionality Plugin?

Hi I am trying to find an e-commerce plugin that mimics Audible.com’s credit system. I want people to be able to subscribe to a monthly plan that will award them a certain number of credites a month and then they can purchase products with those credits. If there is not a plugin then is it possible to this with s2Member or a combination of several plugins such as jigoshop & s2Member? Help would be very much appreciated. My issue is I want the easy of use that a e-commerce plugin provides such as tracking that an item is still in stock and inventory. Even better what e-commerce technologies is audible.com built on?