I know that this post is a FAQ, but this field is so broad that I'd like to use the expertise of this forum to narrow my options to a manageable few. I have done some reading on http://www.mgoldmine.com/
which is a great site for this kind of information.
I must recommend a different payment processor for a client. They are currently with Verisign PayFlow Link but PayFlow Link ($20/mo) does not integrate with my cart in a usable manner. Verisign PayFlow Pro does integrate, but it is $50 or more per month and I don't need the other improvements.
This client does about $1500/mo for 10-20 transactions. The merchant account is set up with FDMS Nashville. My cart supports Authorize.Net, WorldPay, Protx, and Paymate in addition to PayFlow Link. Other payment processors (most) can be added for a $100 option, but I'd like to avoid that if I can (my fees have already been set so the $100 comes out of my pocket). On the other hand, I want to provide a good solution and pay if I need to.
Looking at the Authorize.net site, how do I choose a reseller or other provider? I believe the client does a few point of sale transactions, but would be willing to put the information on a web page if that simplified things.
Authorizenet.com uses First Data (like a lot of electronic gateways in the US) as their transaction processor to help verify the transaction. Authorizenet.com should work for you. As well as LinkPoint and Verisign's gateways.