I've seen several posts with people looking for a shopping cart that can be indexed by the SE's. My question is, why is this necessary? A shopping cart is going to be unique to each customer, so why would you want these to show up in search results instead of just the product pages? Just curious.
I asked essentially the same question in this thread [webmasterworld.com]. The reply is that people are using the phrase "shopping cart" when they actually mean the entire store -- products database and all.
Not sure I understand your question, but I think you'll find that when people are referring to "shopping carts" they mean complete ecommerce solutions, not just the checkout and payment pages. When they ask for recommendations about carts with Search Engine friendly pages what they're effectively looking for is a solution where the catalogue and product pages can be easily crawled.