Retailers and credit card issuers settled last week on terms that will allow vendors to apply surcharges to customers who choose to pay by credit. The additional charge, if implemented, will be used to compensate for small fees the vendor assumes for processing the transaction. This charge can typically range from 1.5 to 3 percent of a purchase when using plastic. And while most merchants are not expected to take advantage of the ruling, don’t expect the ones who do to exercise the same power that several Long Island gas stations have demonstrated.