Shopping carts are more commonly used as a way to display physical products, products like televisions, groceries, electronic equipment, clothes, and memorabilia, etc, not digital products. Shopping carts make it easy to cross-sell and up-sell your customers once they arrive at your store.
Shopping carts typically handle the shipping and handling calculation, taxes and credit card processing. A good shopping cart handles all of these efficiently and securely.

Dynamic Operation

A good shopping cart program also creates dynamic order forms on the fly as the order takes place. The shopping cart should be totally dynamic; meaning it only executes code and retrieves products, images, and product descriptions from your database when your customers request it. During the checkout process it should also calculate shipping/handling and taxes for you.

Real-Time Credit Card Processing

Powerful CGI, PHP, CFM, ASP, etc, scripting, processes commands for you on secure servers when your customers place their orders. Don't get this confused with credit card processing.
That’s mean passing of your customers' order information from a secure form, to your merchant or payment processor, using one of the scripting languages above.
Your payment processor or merchant, like PayPal, 2CheckOut, or Authorize.Net will then process the credit card. If you don't currently have a merchant or payment processor, and you are going to be selling online, then you need one.

Usability

You can choose what options you want your customer to see. For instance, you can choose to display a search form on your shopping cart. A search form allows your customers to search your store for items by keyword or product name, etc.
You can choose to display options like color, size and quantity, and even adjust the price based on the customer's selection. These are called options. Your shopping cart should also allow you to upload images of your products as well.

Stand-Alone –Vs.- Hosted Shopping Carts

You can purchase standalone shopping cart software or services. The stand alone software, of course, requires skill and expertise to install it on your web host or on your own server. Some of the shopping cart services, although it may not seem like it at first, limit you as to what you can and cannot do.
These services are most commonly offered by web hosts as a way to entice you to host your site with them. When using these types of services, remember, you're locked into using that web host when you opt to use their shopping cart service. Your business is not portable and becomes a part of that web host.
Make sure there is a simple export process that allows you to easily download your website and shopping cart if you no longer wish to use their services.
The best option is to use a shopping cart that runs from your own website, independent of your web host. Both Prowebware and stand-alone shopping cart applications run from your website, giving you both control and flexib! ility.

Choosing a shopping cart boils down to 4 things:

1 Your Budget

2 Your Skill-level - Do you have the expertise to edit & install scripts on your server?

3 Desired Functionality - Want a professional results oriented system or a display case?

4 Time - Have time to take away from your business to shape and mold a new program?

Whatever you decide, your cart should also permit you to follow-up with your customers automatically and even instantly auto-subscribe them to your mailing list.

To implement most out-of-the-box applications, you will need to know how to edit PHP, HTML, or ASP code and how to set up a MySQL database on your server for dynamic operation. Then, with some, all you need to know is how to copy/paste some simple code to your existing web pages or the template you're using. The system should also include some type of easy help or instructions, like an HTML file or contextual help menus that walk you through setting up your new cart.