Parched beyond belief, you dive into the nearest convenience store for a cold beverage. You open your wallet and find that you're out of cash. No biggie, you'll just use one of your cards — or so you think, until the clerk tells you that your $1.59 drink isn't enough to meet the store's minimum purchase amount for using debit or credit cards.
You'll often find such informative notes taped on the keypad where you swipe your card, and when you do (and your card is a MasterCard or Visa), you can mention that the store is in violation of its merchant agreement. Merchants cannot impose a minimum, or a maximum for that matter, when you're using one of these cards. You might not be able to convince the clerk of any wrongdoing, but you can take action by reporting the merchant to your credit card carrier.