Before you compare quotes, learn the three parts every card payment is built from. Two of them are not set by your provider at all.
The merchant service charge on your statement is interchange, plus scheme and processing fees, plus your acquirer's markup. Only the last one is a negotiation.
Take last month's statement. Write down the total you paid and the total card turnover. Divide one by the other. That single percentage is what the next five lessons take apart.