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How can I stop a merchant from charging a credit card processing fee?
You can report the violation to the payment network (i.e., Mastercard or Visa).  For instance here is a report form for Visa and here is one for MasterCard.  I just found those by googling; there are no doubt other ways of contacting the companies. Needless to say, you shouldn't expect that this will result in an immediate hammer of justice being brought down on the merchant.  Given the presence of large-scale fraud schemes, it's unlikely Visa is going to come after every little corner store owner who charges a naughty 50-cent surcharge.  It is also unlikely that threatening to do this will scare the merchant enough to get them to drop the fee on your individual transaction.  (Many times the cashier will be someone who has no idea how the process actually works, and won't even understand the threat.)  However, this is the real solution in that it allows the payment networks to track these violations, and (at least in theory) they could come after the merchant if they notice a lot of violations.