Alot of my customers have told me that Costco has a merchant account thing worked out with IONGate. Many of them have switched from various others to this because the pricing is so reasonable. Reasonable enough to pay me to switch the coding for them.
in reply to interchange for e-commerce?
I don't use them, or any merchant accounts so no personl experience, except writing to them. Their interface is pretty direct and simple. You will need SSL if you want anything beyond "Thanks for your credit card" or using their form. But again its pretty basic, of course most of them are now. It doesn't profit them to make it difficult for Perl programmers to interact with them.
Good luck, and umm... do you got any good Witchblades?? :-P