The chief criteria for judging an ecommerce shopping cart are the number of credit card processors and shipping services it supports, and the number of people that support *it*.
Why? Because credit card processors and shipping services mutate all the time. Your cart will require updating. Which service works today may go out of business tomorrow, and leave you with the orders piling up.
Other important criteria are how easy it is to set up, and add products, and how easy it is for the customer to use.
When you set up your shop test it using a wide variety of the oldest and buggiest browsers you can find. If your web store works under them you're home and dry.
Here are the cheapest, simplest, and most effective carts I've found:
Oscommerce (free) - http://www.oscommerce.com
A very good, full-featured, cart. Uses Php and MySQL. Not easy to set up for a 'newbie'. Cookies are used to track the order. If you have PhpMyAdmin installed in your web account, it's easier. Requires a customer to register before they can make a purchase. Supports a wide range of credit-card processors and shipping services.
Bad point: Technical support is limited to the Oscommerce forums, which are not helpful to newbies.
Also, it may be a while before an update is available to a payment module. These are done by unpaid enthusiasts.
Good point: Oscommerce is supported by thousands of unpaid enthusiasts; this means updates do eventually arrive, and it's less likely to go out of business, unlike a commercial cart.
X-Cart (commercial) - http://www.x-cart.com
Similar to Oscommerce. Requires a customer to register before they can make a purchase. Lots of features and add-ons. Supports a wide range of credit-card processors and shipping services. Has an affiliate program add-on, and lets others sell products though your cart.
Dansie Cart (commercial) - http://www.dansie.net
A well specified cart. Supports a wide range of credit-card processors and shipping services.
Bad point: Apparently the Perl code is obscured, to make it harder to copy, which is annoying if you want to customise it.
Interchange (free) - http://www.icdevgroup.org
A version of the old Akopia / Minivend carts. Complex product with lots of files and a lot of setting up to do. A complete solution, and includes the option of third party credit-card real-time order processing. Encrypts orders.
Agora (free) - http://www.agoracart.com
A Web-Store/Commerce.cgi hybrid.
The Commission Cart (commercial) - http://www.siteinteractive.com
A CGI-based shopping cart which also functions as an affiliate program. Other webmasters earn commissions by signing up and linking to your site.
ShopFactory (commercial) - http://www.shopfactory.Com
Has a nice little wizard-based set-up. If you have a lot of items in your shop, this is an option worth checking out.
This is a very valuable feature. A person who's set up CGI scripts before will get the most out of this. Supports a wide range of credit-card processors and shipping services.
WebGenie Shopping Cart Pro (commercial) - http://www.webgenie.com
It's for someone who hasn't set up a CGI script before. Expensive for what you get, but it works. Option to buy it on hire-purchase.
Actinic Catalog (commercial) - http://www.actinic.co.uk
Most suitable if you have lots of items in your store. It's a 'wizard'-based PC program; you type in your information, and the program sets up the store.
You should set up the default store as-is, then customise it later. Supports a wide range of credit-card processors and shipping services. A little complicated for an internet newbie; there's quite a lot in it.
Order Maven (commercial) - http://www.briggsoft.com
A clever little program. It's a standalone Windows executable, which the customer downloads. The customer starts it up, chooses the product, enters their details, and sends off their order like an email, with the credit card details encrypted. It costs $29.00 at the time of writing. No secure server or order page needed; it's all done on the customer's PC at their leisure.
You need to customise it. Make sure you write your mail server URL into the code. For the customer, the order module is a 160kb download.
Having read the above you should have eliminated quite a few programs from your shopping list. They either won't work properly with all browsers, or they won't encrypt your order, or they want to tie you into their manufacturers' secure order system at $40+ per month.
Another option is to get a bespoke system set up for you by a specialist company. These cost thousands of dollars. Aren't you glad you read this far?
About the author: T. O' Donnell (http://www.tigertom.com/web-designer-london.shtml) is an ecommerce and web site design consultant in London, UK.