Know the policies to your specific credit card or debit card in regards to minimum purchases.
*The official policy of Visa: "Visa merchants are not permitted to establish minimum transaction amounts, even on sale items. Imposing minimum or maximum purchase amounts is a violation."
* The official policy of MasterCard: "A merchant must not require, or post signs indicating that it requires, a minimum or maximum transaction amount to accept a valid MasterCard card."

When a merchant posts a sign or explains to you that there is a minimum purchase requirement to use your Visa or Mastercard, politely and confidently inform the merchant that they are violating the policies of said card.

If the merchant still denies the use of your credit card, you can make a dispute via contact infor for each specific card.
*Visa
Phone Number: 1-800-VISA-911 (International: 1-410-581-9994) There is also a customer service number on your card as well.
Online: You may be able to send a complaint via your card issuer's website in regards to merchant violations. You may be able to file a dispute if there were fees unduely charged to you.

*MasterCard
Phone Number: 1-800-MASTERCARD (International: 1-636-722-7111) There is also a customer service number on your card as well.
Online: You may be able to send a complaint via your card issuer's website in regards to merchant violations. You may be able to file a dispute if there were fees unduely charged to you.