Offer your thoughts or opinion on the input financial query or topic using your financial background.

How can I stop a merchant from charging a credit card processing fee?
You have no recourse on the spot to do anything to the vendor other than pay the fee, pay cash, or walk away.  If you're on a mission with longer-term horizon than immediate satisfaction, your options will vary by state.  If you're in a state where the fees are legal and the owner is (potentially) violating an agreement with the card company, you can report the vendor to the card company.  They may or may not really care.  If you're in a state where the fee is actually illegal, you'd need to see what options you have with the local authorities. You should keep in mind that if the vendor is violating an agreement that's between the vendor and the card company only, you have absolutely no rights to enforce that agreement.  You only have legal rights if you're a party to the agreement in question or if the law gives you some special rights specific to given circumstances. (The lawyers call this having "standing.")  Likewise if the vendor is doing something that's not consistent with the agreement between you and the card company, you also have no claim against the vendor (because the vendor is not party to your agreement with the card company), although you might have a claim against the card company.