Raising fees on small-ticket debit purchases could hurt self-service kiosks, such as DVD rental kiosks, which "benefit from debit-card acceptance, " Mr. McCrohan wrote.

WSJ: MasterCard, Visa Seen Raising Fees

At the ticket machine, I slid in my debit card, punched up the route and chose a day return.

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Nexus has so far upgraded 52 Metro stations with smart ticket machines which are capable of accepting credit and debit cards and banks notes, as well as coins.

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Most merchants should see a reduction in their costs for accepting debit cards as a result of the new caps, but small-ticket merchants are "shocked that, in their case, their costs are actually going to increase, " said Mark Horwedel, chief executive officer of Merchant Advisory Group, a trade group that represents large retailers on card-acceptance issues.

WSJ: MasterCard, Visa Seen Raising Fees