Perhaps there is a better option (I would love to learn about it) but the ability to pass data to foxycart.com would be very helpful. While it is not a free service, it handles a wide range of payment gateways(authorize.net, paypal etc) and I think it would be very useful.
Chris Coyier did a couple of screencast that show it off:
http://css-tricks.com/video-screencasts/56-integrating-foxycart-and-wordpress/
http://css-tricks.com/video-screencasts/45-using-foxycart-for-ecommerce/