Reuters reports that Costco is about to drop American Express as their preferred co-branding partner by March 31, 2016.
According to the article, American Express and Costco were unable to find a commercial agreement. I find it astounding that American Express would not continue the agreement. All the investment is in place and the partnership must have signed up a huge amount of new American Express customers.
From April 2016, it is likely that you will not be able to use an American Express credit card at Costco at all – instead MasterCard and possibly Visa credit cards. Costco does accept debit card payments, though.
I was once a very devoted fan of Costco, but was always annoyed that American Express would classify it as wholesale and not groceries, so the 2x grocery bonus would not apply. Oh well – I don't have to worry about this anymore.