Since you are supplying web site design services, I strongly recommend you check out
www.mals-e.com for e-commerce. This is a free service that allows you to collect credit card data to process orders, while maintaining the files with the "buy" links on your site. I've been using the service form 5 years now and it has only gotten better. You need to have a basic underatanding of HTML and that's it. There are examples that lead you through the process and it is not difficult to setup. Check out the Shopping Cart user to user forum on his site to see how others think. Mal Stewart, the creater, actually answers his email and has conducted his business with class.
You can check
www.kepcopower.com to see how I chose to implement it. I know this seems like an advert, but since the service is free there is obviously nothing in it for me but the good feeling of passing on a great find.
Steve Kugler