From your customer's point of view, a web-shopping cart is simply a place to collect one or more items, and then arrange shipping and payment easily and securely.
If you have an e-commerce enabled site, from your point of view, your shopping cart is where your website succeeds or fails.
If your customer abandons her shopping cart for any reason before completing the sale, you've not only lost that sale, but you've probably lost the customer as well. Customers abandon shopping carts for a variety of reasons.
The most typical are confusion about the process and perceived slowness of the shopping cart process. You need to be sure the shopping carts on your website work quickly and easily each and every time a customer uses them.