[takeda64]

AmEx has special Costco card, which you can only use with valid Costco membership (it automatically renews it per year) and the cashback you are receiving will be in a form of coupon given yearly which you can only spend back at Costco.

[taftster]

You can cash the check at the customer service desk. But yes, you can only redeem the cash-back coupon at Costco.