I received a check back in Nov. 2013 with a letter referencing Costco AMEX True Earnings rebate (not AMEX travel...), contact: 1-855-720-6799.  Costco AMEX does have rebate checks which can be redeemed at Costco, but this letter specifically said "Please do NOT bring it to Costco as it can't be redeemed or cashed".  The check has US BANK, Miamisburg, Ohio, routing number (042205038).  I called US BANK and they indicated the routing number I am asking about is NOT for any of their cashier or money order checks and if it doesn't match their website which lists routing numbers for the Ohio area, I should bring the check into one of their branches to verify if it's real.

I just received a letter with a refund check attached from American Express. I have no idea what it is in reference to, but the phone number they list to call for questions is 1-855-720-6799.

Letter state this program is managed by American Express Travel Related Services Company.

Hope this helps!