Costco cardholders will no longer be able to pay with American Express come June. That’s when a deal for American Express to sell its Costco-branded portfolio to Citibank is expected to close, and cardholders issued new plastic. The agreement has been in the works since Costco announced it was cutting ties with American Express last year. Citi was originally supposed to take over from American Express April 1, but the deal was delayed. A specific date is still to come, but once the transition is official, the American Express Costco cards will no longer work, says Elizabeth Crosta, a spokeswoman for American Express.