(The.Comedian) Five months after American Express and Costco announced they would go their separate ways and end their exclusive relationship – essentially allowing members of the warehouse club to use other cards – shareholders for the credit card company have filed a lawsuit claiming it blindsided investors with the loss of the contract. The lawsuit [PDF], filed by Plumbers and Steamfitters Local 137 Pension Fund, accuses American Express of failing to reveal how significant the 16-year long contract with Costco had become to the business. According to the lawsuit, which seeks class-action status on behalf of all shareholders who bought stock between Oct.