This article is from the Credit cards and Consumer Credit FAQ, by firstname.lastname@example.org (Steve Adams) with numerous contributions by others.
If merchants violate any of the above laws, you can report them to
your state's or city's consumer protection office or attorney
general. If they violate any rules of American Express, the company
would like to know about it. Report violations of Visa or
MasterCard rules to the bank that issued your card; if the sale was
completed, and you can also send a letter with a copy of the charge
slip to the Visa or MC address given earlier in this section.