Jan. 26, 2013 -- Starting Sunday, paying by credit card could get more expensive. Under the terms of a $7.2 billion settlement reached last summer between credit card companies and merchants, merchants will be free to impose a surcharge on customers paying by credit card.

How big a surcharge depends on how much the merchant pays in processing fees, but the amount legally permissible will be between 1.5 percent and 4 percent of your purchase price.

No one knows how many merchants will exercise this right, but Gerri Detweiler, director of consumer education at Credit.com, expects the number to be small, at least at first.

READ MORE: Credit card settlement may not help consumers.

Smaller merchants, she says, typically feel gouged by processing fees and are more likely than big chains to pass the cost along to their customers. Service providers, she says—your accountant, your massage therapist—are the most likely to pass the charge along. Among big retailers, however, only gas stations have historically distinguished between cash and credit customers, offering a discount to customers paying cash or imposing a surcharge for those using credit cards.

Smaller merchants, says Detweiler, don't always know what they're paying in processing fees. "If you think your own credit card statement is confusing," she says, "take a look sometime at a merchant's credit card agreement with VISA or MasterCard." If she herself were a merchant, she says, she'd be hard pressed to figure out the right percentage to pass along to customers. "I wouldn't want to wade into those waters," she says.

By law, merchants intending to pass the cost along will have to post notices at check out informing consumers of the extra charge. Online merchants will have to post a similar notice to their home page.

Ten states prohibit credit card surcharges, so if you're making a purchase in any of the following, you won't have to worry about being penalized: California, Colorado, Connecticut, Florida, Kansas, Maine, Massachusetts, New York, Oklahoma and Texas.

READ THIS: The Low-Down on Checkout Fees.

You also don't need to worry if you're paying by debit card, since those are excluded from the settlement agreement. Nor do American Express customers need worry: AmEx's contract with retailers forbids them from levying a surcharge.

Detweiler offers this advice to affected consumers: "Always have a back up method of payment," she suggests, so you can avoid paying the new charge. "Have a debit card, or slip and extra $20 in your wallet."

And also, she suggests, tell your merchant if you object to the new charge. "If enough consumers complain, a merchant will fear losing business and won't choose to pass the charge along. I don't think people are going to like being penalized for paying the way they want to pay."