OK, I got it. Just looked at the Authorize.net rate offered by
Merchantplus: $9.95/month and $0.05 per transaction (1st 250 transaction free/month). Great.
The guy at
Merchant plus told me that they don't require a long term contract but their Issuing Bank does require a 2 year contract, so basically it is a 2 year-contract. However he added that if I need to cancel it, just give them a call and the account will be canceled with cancellation fee waived. It is no problem for me.
If I register with Merchantplus (of course with my real name & address) and integrate it Auctiva through Authorize.net, the checkout process will occur at Auctiva's website, and I should be safe then. Am I right?