Shopping Cart OScommerce or Zen Cart

The main purpose of the shopping cart is to enable "club" members to select a free items from a catalog. Here are the specifications: A. First and foremost, it has to be easy for our customers to use. The interface should be clean with a minimal number of steps/screens in the ordering process. Think Amazon. B. The site is to be accessible to subscribers only. C. All items are to be sold for free. D. Each subscriber can only purchase (for free) the quantity of items determined by the coupon code that was emailed to him/her. E. Inventory should be depleted as items are sold. If inventory is zero then the item can not be purchased. F. Only several categories required. Each category will contain numerous pages of 100 items each. G. We need an administrative back-end that gives us the ability to: 1) Easily add/edit/delete products ourselves. 2) Add/edit/delete subscribers. 3) Add/edit/delete coupon codes. H. The cart must be programmed in PHP (along, of course, with mySQL). That's about it. Once you're finished installing the cart, we have about 20 products we'll want you to add to it (all content for each item has already been written).