If you’re shopping at Costco, check your wallet — the retailer has stopped accepting members’ American Express cards.Monday marked Costco’s official switch from its exclusive partnership with American Express to Citigroup and the first day the wholesale club accepted only Visa credit cards, whether the co-branded Costco Anywhere card or any other Visa card.Costco members who had one of the club’s American Express co-branded cards should already have received their new Costco Anywhere card in the mail, and the old cards will automatically be canceled, Costco said.A “really, really minimal” number of shoppers told Costco they didn’t want the new card, most likely because they already have a Visa card, said Costco spokeswoman Bev Akada.Monday also was the first day Costco members who did not automatically receive one of the new Visa cards could apply for one at a Costco warehouse or at citicards.com.