Because Costco only accepts American Express and this card is a MasterCard, there is a chance that the card transaction will decline. However, there have been card transactions that have been approved at Costco – most likely from the Costco Pharmacy. The best way to find out is to attempt to use the card. If it declines, you may pay with a different form of payment (cash, debit card) and submit a claim for reimbursement to TDS.
125 Plans/ Debit Cards: Does my card work at Costco?