There is never a minimum charge required for Visa or MasterCard and generally not for American Express. Although this is not actually stated in law, it is part of the agreement with Visa or MasterCard. According to Bankcard Holders of America, if a merchant accepts American Express and Visa or MasterCard, American Express does not allow the merchant to impose a minimum purchase on American Express users. This would discriminate against the American Express users. However, merchants who accept American Express, but do not take Visa or MasterCard may impose minimum charges although American Express officially discourages this practice.

Source:http://www.credit-factor.com