This article is from the Credit cards and Consumer Credit FAQ, by email@example.com (Steve Adams) with numerous contributions by others.
Never for Visa and MasterCard; generally not for American Express.
Discover explicitly allows the merchant to set a minimum purchase
amount, according to email received by the previous editor.
MC and Visa rules provide that a merchant may not require any
minimum purchase amount. This is the merchant's agreement with Visa
or MasterCard; it is not a Federal law. (On the other hand, if you
insist on charging a 79-cent ball point pen, I hope you get four
flat tires on the way home.)
According to Bankcard Holders of America, if a merchant takes
American Express and also Visa or MC, Amex doesn't let the merchant
impose a minimum purchase on Amex users because that would
discriminate against them. Merchants who take Amex but neither Visa
nor MC may impose minimum charges but Amex officially discourages