364120 no checkout shopping cart

Hi I need a PAINFULLY simple shopping cart. This cart should be the type (like html cart or CARTcgi) that allows pasting of code into regular html pages. Here are the specs. I'm doing my best to communicate this as simply as possible, and hopefully I've covered all the bases. 1. We will have individual page for each category - and on each category page there will be several items for sale, each with a "buy now" or "add to cart" button. 2. When buyer clicks "buy now" or "add to cart", the item will be added to cart, but NOTHING WILL HAPPEN -- it is VERY important that clicking buy now does not move the buyer to another page. (normally, upon clicking buy now, buyer is taken to a product page where he has choice of continue shopping, checkout, etc.) 3. However ... in the upper right corner, we'd like to have a typical "cart contents" update like [login to view URL] 4. There need NOT be any "checkout" link. What I would like is to place a "request invoice" link which will take them to instructions on how to do just that. I can handle this part. 5. The pages MUST be my own regular static HTML pages, and my client should be able to add products to a category page using a typical web/text editor - center around giving me an installed script, with instructions for install for future, and it should operate by placing code snippets on my already designed product pages (much like you'd paste code for paypal button) 6. under no circumstances should buyer have to leave the site.