A new rule going into effect Sunday could cost you more when shopping with a credit card at some stores. Visa and Mastercard have agreed to let merchants add a service charge equal to the cost of processing a credit transaction to the bottom line. The cost of processing is usually 1.5 to 3 perc cent, and merchants are capped at a 4 per cent fee under the agreement. The rule change was made as part of settling an antitrust suit brought by retailers. Merchants will still not be allowed to add a surcharge to debit card transactions. However, few...