>We have a client that would like to add credit card processing to their VFP 9.0 application. What is the best way to accomplish this?

Take a look at this article:

http://www.west-wind.com/presentations/aspnetecommerce/aspnetecommerce.asp

It's not VFP specific but the same concepts apply and you can actually easily adapt the code for VFP.

It explains the process of getting a gateway provider and processor as well as the actual implementation against a few providers like Authorize.NET, PayFlow Pro etc.

Hope this helps,