I'm offering a bounty for helping me edit the checkout script to integrate with the FastTransact merchant account, since they offer Authorize.net compatibility and are already supported by a few other shopping carts.

That's why I'm gonna make it as easy for you guys to do this as possible, since I DO NEED IT ASAP (as in the next few days, if possible!)

Here is the documentation from FastTransact regarding their integration with other shopping carts:
https://secure.fasttransact.com/public/docs/merchant/public/shoppingcart...

And then on compatibility with Authorize.net:
https://secure.fasttransact.com/public/docs/merchant/public/nativespecs/...

and all other docs needed (if at all):
https://secure.fasttransact.com/public/docs/merchant/

Anyone interested in tackling this one, there's a bit of money in it for ya if you can do it quick. Post any questions you have in here.

Thanks!