A popular business practice with credit cards is to set a minimum purchase amount. But that violates a little-known rule.

Merchants pay sometimes steep fees for accepting plastic -- and it makes a difference on small purchases. But the major credit card companies prohibit setting minimums.

You see the signs all over -- if you want to use plastic, you have to spend more than a few bucks.

"A lot of people were charging 89 cents, 79 cents, and I just said no," said Martha Sanchez, owner of Mercado de la Raza.

That's because taking credit cards takes a swipe out of profits.

"It's not free. It's nowhere near free," said Carol Pregill of Retail Merchants of Hawaii. “For smaller companies, something as much as 4% taken out of that transaction is a big bite."

A big bite merchants are required to swallow.

Visa and Mastercard rules state that any merchant accepting credit cards cannot set a minimum purchase. American Express and Discover allow them, but not if other cards accepted on premises don't. Vendor's say it's a bad rule because it can make them lose money on very small buys.

"I know that technically I'm not allowed that,” Sanchez said, “but when I see my fees that I pay every month, I said I have to set a minimum, because otherwise I'm giving things away."

Vendors pay a percentage of every purchase, plus a per-transaction-amount and other costs like statement fees. Retail experts say it's just a cost of holding onto the growing number of credit card shoppers.

"You're in there to do business -- you're operating to do business, and sometimes you just have to bite that bullet," Pregill said.

Other stores that don't set minimums say they'd still like cash for a smaller buy.

"We would ask them first if they did have cash,” said Ravy Winter of Asian Grocery. “It would make it a little easier on us."

But they say most credit card shoppers spend more -- and it's a reliable payment. Both Mercado de la Raza and Asian Grocery stopped accepting checks because so many customers bounced them.

"With the credit card you have the peace of mind that it's approved,” Winter said. “You don't have that with the check. I'm paying for peace of mind, and I guess so is the consumer."

Vendors can offer cash discounts to promote cash on purchases of all sizes. But at a store taking plastic, consumers have the final choice on how to pay.

If a complaint cannot be resolved with a merchant, here’s how to reach the credit card companies:

Visa USA Consumer Relations, askvisacorporate@visa.com, or “If you run into a problem like this with a merchant, please notify the financial institution that issued you your Visa card.”

MasterCard International, 1-800-300-3069

American Express Customer Service, 1-800-297-1234