I think you're misunderstanding what an off the shelf shopping cart is. It's not just a way of storing the items a user has chosen along with their quantities, like you say, all that requires is a session array.<p>It's product management and categorising, product options, coupons and discounts, multiple tax rates, bulk buy discounts, user registration, payment gateway integration, promotions, image galleries, I could go on.<p>Basically when you buy an off the shelf system like zen cart, you're getting an entire e-commerce site that you just configure and style. And even if it's not a full site, but a PHP library to interface with your own site, it's a lot more than just the action of storing product id's and quantities in a session.