I came to ask the same questions about Authorize.net and was ready to use it along with Zen-cart. I just read on the site "the merchant must have a U.S. based merchant bank account" So now I guess I can't use it. Does anyone know otherwise?
Authorize.net has good customer service and are a lot cheaper than Pay Pal. There are so many fees with Paypal the only plus is people tend to trust Pay Pal. Best of Luck