Costco does have a special rate for accepting JUST Amex besides cash and debit (For clarification I mean when you purchase items at Costco not the merchant service they describe ). The normal rate for small brick and mortar merchants for Amex is a solid 3.5%. Visa,MC,Discover can easily get to 1 to 1.5% if you do any volume and are brick mortar. Online and phone rates are ofte higher because of the higher chance of fraud, disputes, etc. Any merchant can have there rates jacked way up if they get a lot of chargebacks. They can even be blacklisted so they can not get a merchant account. These scum merhcants just set up under another entity.

Also not widely know both Costco and Sams club let online portals accept other forms of payment than stores