For years, Costco has exclusively accepted American Express credit cards. Well, as of March 2016 that will no longer be the case. Amex and Costco were unable to reach a deal regarding their exclusivity agreement. Therefore, come next March, Costco will no longer accept Amex card but instead will begin to accept Visa credit cards. I can't see how this is not a blow for Amex. I don't think people will stop shopping at Costco because they can't use their Amex card. However, I think there are many people who only have an Amex card because of the relationship with Costco. Costco Amex cards account for 10% of all Amex cards.