> except Costco where I have to use Visa and I have their card for that
I imagine this is due to some dispute with Mastercard on their fees for credit cards (you can use MC debit at Costco); Visa probably gives Costco a steep discount on fees.
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.)
I imagine this is due to some dispute with Mastercard on their fees for credit cards (you can use MC debit at Costco); Visa probably gives Costco a steep discount on fees.