The Summary of Rates and Fees explicitly lays out how foreign transactions are charged. https://www.cibc.com/content/dam/cibc-public-assets/personal-banking/credit-cards/all-credit-cards/costco/documents/summary-rates-and-fees-en.pdf Foreign currency Transactions (except CIBC Convenience Cheques) are converted to Canadian dollars on or after the Transaction date and no later than the Transaction posting date. You are charged the same conversion rate CIBC must pay, on both debits and credits. You are charged a fee of 2.5% of the converted amount, on both debits and credits. This fee is charged on currency conversion. A foreign currency Convenience Cheque or payment is converted at CIBC’s branch selling rate for Canadian dollars in effect at the time it is processed by CIBC. To reiterate, CIBC charges you what the $CAD amount is after Mastercard converts it, then they tack on an additional 2.5% fee on top. So you're effectively paying 2.5% more than if you use a no-fx fee Mastercard, like Brim, HSBC, Wealthsimple Cash or EQ Bank Card. CIBC is not giving you any discount. More on reddit.com

I have used my Costco MC many times at Costco in the US. It works nearly all the time if you insert it, it may not work when tapped. Costco US staff may not be aware that the card is accepted and if they see the MC logo they may try to stop you from using it. But the system is configured to accept it. If you have a no-FX Visa, you are probably still better off using that as any cashback on the MC won't offset the 2.5% forex fee. In that case, still no issue using your Costco MC as your membership. Another good option is to buy Costco Cash cards in Canada and then use them in the US. The exchange rate will be similar to a no-FX credit card. More on reddit.com