Getting ecommerce sites to convert with a shopping cart.

Just about everyone that has a brick and mortar these days, also has an online business storefront as well. These online stores always have some sort of checkout process that ranges anywhere from 3-20 steps to complete to get the purchase completed.
You really should find a happy medium to accommodate the variety of users you may be experiencing. Part of your marketing campaign "research and development" you should already have a great idea of who your target market is and you should really design and develop your site if it already isn't, for that particular demographic.
When choosing your shopping cart method, or when you develop it yourself keep in mind the number of steps and the information that you may require. The acronym "KISS" (keep it simple stupid) still holds strong today and really should be implemented on the web as well. Keep your checkout short and sweet and make sure all your charges are clear cut. Shipping charges might make a few people drop from the process, but better to have them drop early so that you could pinpoint that your shipping charge is what is stopping the sale. Unfortunately with Ecommerce sites there is no salesman that is going to inform you where he lost the sale, so you have to use behavior and bounces to determine which step is causing you to lose sales and move from there.