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Which credit card is friendliest to merchants?
Back when they started, Discover undercut Visa and Amex fees by about a point. This was also true when I worked for a mail-order computer retailer in the '90s: if a customer asked us which credit cards we took, we were told to list Discover first (and AmEx last) because Discover had the lowest merchant charges. Possibly this is no longer true today, but for quite a while it was a significant selling point of the Discover card to merchants, and a reason why many did sign on. (A reason some stores did not sign on was that Discover was owned by Sears, and many businesses that competed with Sears didn't like the idea of sending any of their profits to the competition.) Today, Discover also owns Diners Club and the fees for those cards are higher.