Unfortunately, for those of us who are Costco members and would like to shop/buy gas in both Canada and the US, the acceptable credit card is DIFFERENT: MasterCard in Canada and VISA in the US.
We had the reverse problem when trying to buy gas at the Augusta, GA Costco. We don't have a VISA card, and as the OP states, Costco gas bars do NOT accept cash.:(
A way around this problem if you have the time is to go into the store and use cash to buy a "Costco Cash Card", which you can then use at the gas bar.
It really is too bad that Costco Canada and Costco USA don't use the same credit card.