Jason Scherr had a lot on his mind the day after he opened his fifth Think Coffee shop in Manhattan last week. The fan was blowing too hard, the classical music was playing a little too loudly—and he was trying to figure out how to get more customers to pay with cash. A new law that was supposed to reduce costs for merchants that accept debit cards has instead sent Mr. Scherr's monthly processing bills much higher and forced him to reassess the way he does business. "My choice is to raise prices, discount for cash or get an ATM,"...