> I imagine this is due to some dispute with Mastercard on their fees for credit cards

Costco has long had an official credit card and only allowed cards of that brand (whether theirs or not), for a long time it was Amex not Visa.

> Visa probably gives Costco a steep discount on fees.

Right; their single-card-brand partnership agreement is a tool for getting lower credit card processing fees.

> Right; their single-card-brand partnership agreement is a tool for getting lower credit card processing fees.

Volume rates in general are extremely common.

> Volume rates in general are extremely common.

This combines “volume” with “exclude our competitors”.

My point (theory) is that Mastercard isn't offering them a big enough volume discount. If they could pay 0.1% I doubt they would exclude MC even if it meant losing out on some Costco credit card accounts.

I'm saying excluding other card brands gets them a bigger discount from their exclusive card network than volume alone would, not that it pushes people to the Costco-branded card from that network (though it probably does that, too.)

In Canada, I believe you can only use Mastercard at Costco, interestingly.