Surely the grand total of Costco’s US credit-card fees can’t amount to more than a percentage-of-a-percentage of all credit-card spending in the country?
...otherwise WalMart and other major retailers (Amazon?) would have partnered with a single credit-card network to benefit too
Walmart and Kroger have had their own disputes with Visa (for one example) and (at least Kroger) started blocking some of their cards at select chains in order to negotiate a better deal. Costco has super tight margins. The the point that when their execs are talking about the price of goods they don't say, "[Item] is $70." No, they say, "[Item] is 69.99." Their culture is about every last penny of the margin. Their money comes from memberships.
...otherwise WalMart and other major retailers (Amazon?) would have partnered with a single credit-card network to benefit too