The role of payment cards in modern economies can hardly be underrated. In 2013, debit and credit card transactions represented roughly half of all consumer payment transactions in Western Europe, and accounted for more than 45% of all payment transactions in the United States. 2 The payment card industry has faced intense antitrust scrutiny on both sides of the Atlantic, partly in response to the merchants' recurrent complaints about high transaction fees. In the past, regulation has mainly targeted the fees imposed by payment card networks. For instance, the most commonly used regulatory instrument caps the "interchange fee," which is the payment made by the merchant's bank (called the acquirer) to the cardholder's bank (the issuer). This...