Tom,
Over the years I have experimented with all the various forms of PayPal and Auto Return has it's uses but I would never rely on it, but I agree Authorise.net, PayPal Pro etc. is the way to go.
Watercolor,
If you are unsure how to proceed my best advice would be to hire someone who understands how the payment gateway works and get them to complete that part of the project on your behalf. You are looking at a world of hurt if you get it wrong, trust me.