From the Visa website:
Visa merchants are not permitted to establish minimum transaction amounts, even on sale items. They also are not permitted to charge you a fee when you want to use your Visa card.
If you run into a problem like this with a merchant, please notify the financial institution that issued you your Visa card. These institutions have access to the appropriate Visa rules and regulations and can help you document and file your complaint. You'll find their address and/or telephone number on your Visa statement. Their telephone number may also appear on the back of the card itself.
Many small retailers complain that high fees force them to take these measures. I don't understand why they haven't factored those costs in when pricing their merchandise. As a consumer, it's not my responsibility to worry about a merchant's cost of business. All of that should be kept behind the scenes and not interfere with my transaction. Not only is it unprofessional, but it will also turn off a lot of shoppers that put small purchases on their cards to earn reward points.
For your reference:
Visa, Mastercard: Minimum purchase requirements and fees are not allowed. Call the number on the back of your card to report the merchant.
AMEX, Discover: Minimum purchase requirements and fees are allowed as long as they are the same for all card types.
Note: These rules apply for both credit and debit cards. For more information, see the following articles: