There are a lot of factors besides cost to consider as you decide on where to open up your merchant account. How much is Provider A going to charge you compared to Provider B or Provider C is important, but it is not the only issue.
Another factor that you need to consider is service. Some credit card processors may only provide online assistance. They may even say right on their assistance page that they will get back to you in 24 hours. If you cannot call your provider and talk with a real live person on the phone when there is an immediate problem to solve, then you might want to sign up elsewhere. If a customer is waiting and puzzled about why his card didn’t go through, you need quick answers.