Because of the complexity involved with developing an ecommerce web site, it is easy to get tied up with the technical intricacies of integrating or developing a shopping cart and put the customers's experience in second place.

The percentage of shopping carts that are abandoned on ecommerce web sites before a potential customers makes it to the home stretch range between 30 and 60%, depending on the research you read.

Why does this happen? Where does a customer lose trust, or decide that your product is not worthy of their finances? A shopping cart is created when an item is put into it, and by that time the customer knows that they want to buy something from your web site. So why does a shopper change their mind? This article covers a few of the most common mistakes people make with ecommerce usability.
The Shopping Cart Works - That Must Be Enough To Run My Ecommerce Site

Do you think an ecommerce shopping cart 'works' when a customer can technically go through the checkout process and complete a purchase? You might be making a mistake here. Just because it is possible for a customer to go through a checkout process on a shopping cart, doesn't mean that they will. There are too many other elements that can get in the way of their shopping experience to believe that simply having a functional shopping cart is enough to run an ecommerce site.
The Little Lock Means A Lot - SSL and Shopping Carts in Ecommerce

As the public's awareness of the connection between identity theft and online shopping has grown, so has awareness of how to identify a secure shopping cart on an ecommerce web site.

Secure web sites use something called an SSL (Secure Sockets Layer) to encrypt information as it is passed back and forth from a web site to a customer's computer. Browsers will indicate if an SSL is being used on a web site by displaying a small lock on the bottom of the browser window. This lock means that it is very difficult (though not impossible) for computers relaying information between the customer and the ecommerce web site to siphon off meaningful data like credit card numbers supplied to the shopping cart.
An Easy Shopping Cart Checkout Process Means More Ecommerce

Too many shopping carts complicate the customers checkout process beyond what is absolutely necessary. Customers are using your ecommerce web site because it is more worthwhile for them to shop at your store rather than a competitor's, and one way you can stand out is to make your shopping cart as simple as possible.

Often, a shopping cart checkout process consists of several pages, each gathering a different bit of information such as a shipping address, a billing address, credit card information, a review of the order and finally confirmation of the order. This process can be confusing for some ecommerce shoppers and simply too cumbersome for others.
Try to Make the Shopping Cart Checkout Process a Single Page

Source - impliedbydesign.com