Be very careful with authorize.net! They ask you to sign a document containing your credic card details and send it via email.
Then they charge you whenever they like.... I strongly recommend that you should NOT send them any signed document. If you did it, BLOCK YOUR CREDIT CARD as soon as possible, or they WILL CHARGE YOU SOON! This behaviour is totally unfair, don't trust such companies.
Authorize.net is part of CyberSoource that is controlled by Visa.
I'll also block all my VISA cards and only use mastercard....
I strongly recommend 2checkout.com as payment gateway. There are no fixed fees, only fess per transaction, and it's easy to set up and working well.