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I have never seen a location that accepted less than two networks. Though I have never been in a Cosco either so that goes to show how hard I am looking. Generally a business that advertises only the major two will not know the extent of their contracts and equipment. I spend a bit of time teaching servers, secretaries, and other people that handle card machines all over the place.
At one time BJs was like Costco, but only took Discover instead of (at the time) AmEX..... Some examples do still exist.. just not many of them.....
-J