At Gus Miller's newsstand on Forbes Avenue in Oakland, there's a small, handwritten sign taped to the counter. It reads: "$15 minimum for credit card, 50-cent fee for debit."

Kim Gresco barely acknowledged the sign as she reached for her wallet. The cashier rung up her bottle of Pepsi and a pack of cigarettes, little more than $5 total. She handed him her PNC Bank/Visa debit card.

"You know about the fee, right?" he asked.

"Yeah, it's fine," she said.

The transaction may have cost Ms. Gresco an extra 50 cents, but it could have cost store owner Brian Weiss $50,000.

That's because most credit card companies don't allow stores to set policies like the one described on Mr. Weiss' sign and scores of similar notices in convenience stores, newsstands and bars across the country.

Several months ago, after one customer complained to MasterCard, the company through which Mr. Weiss received MasterCard service turned off his credit card machine and threatened him with fines and a lawsuit.

MasterCard, Visa and Discover all prohibit merchants from charging additional fees to use their cards or refusing to accept cards for purchases of less than a set amount. For Discover and MasterCard, this also applies to debit cards that carry the company's logo.

The rules are spelled out in agreements and operating regulations that merchants are supposed to abide by when accepting a particular brand of card.

Ultimately, Mr. Weiss was able to avoid a shakedown and continues to accept MasterCard through a different service provider. He also continues to require a $15 minimum and a 50-cent fee at each of the four convenience stores he owns in Pittsburgh.

But he knows that he's breaking the rules and that more threats of legal action and service termination are always looming.

Why does Mr. Weiss refuse to change his store policy when the risks are so substantial?

His father, Eddie Weiss, who ran the chain of stores for 30 years until his son bought the company, said it's simple: If they don't charge a fee and have a minimum, they don't make money on an average sale.

"They basically say you can't charge additional fees and you can't do a minimum, but we have to pay a transaction fee, and then we have to also pay a percentage fee on the sale," he said.

In the case of a convenience store, where the vast majority of purchases are inexpensive, those fees and percentages can eat up what little profit the store owner stands to make from the sale.

"If somebody buys a pack of cigarettes and a bottle of pop [with a card] we make absolutely nothing," Eddie Weiss said. "If I make 40 cents on a bottle of pop, I'm paying anywhere from 17 to 35 cents just for the transaction, plus anywhere from 1.69 to 4.5 percent depending on the card."

By charging a fee for debit cards, he said, they're only trying to break even on the fees and make a normal profit. Since fees and percentages are calculated per transaction, not per item sold, a $15 minimum purchase for credit cards ensures that the profits from the sale of several items at once won't be eaten up by the fees.

The credit card companies say that policies like Brian Weiss' aren't fair to consumers, namely those who carry their cards.

"We believe using a credit card is a choice and a convenience. Requiring a minimum, we believe, is a form of discrimination," said Leslie Beyer, a spokeswoman for Discover.

Tristan Jordan, a spokesman for MasterCard, said that his company prohibits store fees and minimums because those practices interfere with the prices marked on items in the store.

"It's important to us that the consumer knows what the cost is when they use the card," he said. "We don't want a situation where the consumer goes to the point of sale and is charged a surcharge. That's not fair."

Ben Popken, editor of Consumerist. com, a blog that covers consumer and personal finance issues, said that credit card companies make the rules because want they want the use of their card to be as seamless as possible.

"They want their card to be viewed as just as convenient and just as liquid as cash," he said. "They're against anything that establishes a hierarchy or any distinctions between cash and their card."

Credit card companies also say they provide a suitable remedy for merchants who feel like they incur less expense when purchases are made with cash.

Visa, MasterCard, Discover and American Express allow merchants to offer a discount on purchases made with cash.

For example, a convenience store could knock 50 cents off a purchase paid for with cash to entice customers to use cash. A merchant cannot, however, take 50 cents off purchases made with a Visa card to entice customers to use it in favor of MasterCard.

Brian Weiss conceivably could raise the price of every item in his store to a level where he could make a profit on it and cover the fees if a customer chose to purchase the item with a credit card. If they chose to pay with cash, he could roll back part of that additional markup in the form of a discount.

That scenario makes credit card companies happy, because it keeps the prices customers see on the shelf the same as the prices they pay at the register if they use their cards.

But it upsets store owners, who say that storewide price hikes make it look like they're trying to gouge their customers.

At the 7-Eleven three blocks down Forbes Avenue from Gus Miller's, the management applies its 35-cent cash discount concept with some clever wording.

Their signs, taped to each register and behind the counter, read: "There isn't a premium for credit cards, we do give a discount for cash. The prices you see in the store have the cash discount built in already. So if you pay with a credit card, you pay more (.35) because you don't get that cash discount."

Close, 7-Eleven, but still in foul territory. Because the price a credit card user pays at the register is more than the price on the shelf, the policy violates the credit card companies' rules.

Alden Davidson, who has worked as a cashier at the 7-Eleven for two years, said the policy has been in place for a year and so far the store hasn't been threatened with fines, lawsuits or deactivation.

Ms. Davidson said the store shelled out more than $33,000 in transaction fees last year and that the owner felt like he had no choice but to enact such a policy.

Mr. Popken said rules that prohibit merchants from charging a fee or requiring a minimum purchase drive up prices for credit card and cash users alike.

"In a more open system, if [merchants] were able to pass these fees on to consumers, it would be OK," he said. "Since they can't do that, it increases the overall cost, so they effectively have to do a price increase on every item in the store.

"It's inflationary because the merchant doesn't know if someone is going to pay with credit or cash, and they can't pass on the costs to people who use credit cards, so they ultimately raise prices for everyone."

Readers frequently discuss this issue at Consumerist.com, debating the legality and the morality of both sides, Mr. Popken said.

"It seems to be pretty evenly split. Some people are pretty hard-core and they resent any kind of limit on their action as consumers," he said. "Then there are people who say: 'You're mainly going to see it at a mom-and-pop or smaller stores because these credit card companies charge a fee or a percentage.' "

After just using a credit card to purchase a drink and a sandwich from Ms. Davidson at 7-Eleven, Charles Kennair said he understands what the store is dealing with, and though he finds fees like these annoying, he'd never call Visa and turn in the store.

"What I think that consumers don't realize is how the [transaction] fees are scheduled and how aggressive the fees really are," said Mr. Kennair, who is working on his M.B.A. at the University of Pittsburgh. "[This 7-Eleven] is a small operation, and I find that if they're passing it through and making the consumer aware, I'm all about that. I want people to be aware.

"The point is it's hard enough to squeeze the profit margin. [Small store owners] don't have the kind of billions in reserve that Visa has. Am I going to turn them in? No. I really don't want to spend the time in my day to do that."

Michael Butler, who also used a card to pay for his snacks at 7-Eleven, said that credit card companies who say their product should be no different than cash are out of line.

"I tend to side with the small business owners," he said. "Credit card companies, the way they do business is designed around people not treating it as cash. They make their money when people spend beyond their means."

For Eddie Weiss, this issue is symptomatic of the way the credit card system works in America.

"[Credit card companies] have lobbyists, they go to Washington or wherever and they get all these laws passed and it's good for them and bad for everybody else," he said. "Especially bad for people like us in convenience stores who are working on very small profits. You run out of business by just refusing, because everyone now is carrying around credit cards.

"At the end of the day we know Visa and MasterCard don't allow it, and what the [heck] are we supposed to do? Here we are just trying to get by and make a living, not make any extra money, just trying to be able to take care of our customers."

Ms. Gresco said she sympathizes with Mr. Weiss and she tries to think about carrying cash to avoid paying extra to use her card. But when she tries to get cash from Gus Miller's in-store ATM, it seems to her that it can be hard to avoid fees no matter what you do.

"I usually go to an ATM for cash, and they have one in there, but I don't want to pay a fee!"