It could soon cost you more to shop with a credit card at some stores. As of this Sunday, Jan. 27, merchants who accept credit cards issued by Visa and MasterCard will be allowed to add a service charge to the purchase price. The advocacy group Consumer Action has published a booklet on credit card checkout fees. It warns shoppers to be on the lookout for these fees and advises them to express their dissatisfaction.“Customers shouldn’t stand for it,” said Ruth Susswein Consumer Action’s deputy director of national priorities. “Our advice is to tell them you don’t like the fee and this makes you want to take your business elsewhere.”