From one of my saved Favorites, due to an issue with a motel I was staying at....
MERCHANT CREDIT CARD ABUSES: What They Cannot Ask You To Do
MINIMUM CHARGE REQUIREMENTS
Some stores require consumers to spend at least $20 (or some other minimum) to pay for purchases by credit card. They engage in this practice because they and their banks do not want the expense of processing a credit card transaction involving a small amount of money.
This practice defeats one of the major purposes of credit cards—convenience—and may force credit card users to spend more than they want to. In addition, minimum charge requirements vary from merchant to merchant, and there are no regulations requiring disclosure of these minimum purchase levels.
Visa's and MasterCard's regulations prohibit minimum charge amounts. American Express's regulations do not explicitly prohibit minimum charges, but its policy is to discourage any merchant practices that create a "barrier to acceptance." Amex does prohibit "discrimination" against the Amex card, however, so if a merchant has no minimum charge for Visa and MasterCard, the merchant may not discriminate against Amex by imposing a minimum charge.
TIP: If a store requires a minimum purchase for Visa or MasterCard, point out to the store manager that the practice is prohibited by the card companies.