When the federal law that barred merchants from slapping a surcharge for credit card transactions on top of their “regular prices” expired in 1984, New York enacted a statute — Section 518 of the General Business Law — that says: “No seller in any sales transaction may impose a surcharge on a holder who elects to use a credit card in lieu of payment by cash, check or similar means.”Steamed about the 2 to 3 percent fee they have to pay credit card companies, several New York merchants …