In her 35 years running a small business, Maria Kirk dealt with only one company that made her physically ill: fuel-card provider FleetCor Technologies Inc. and the thousands of dollars in fees it charged.

“Many companies try to nickel-and-dime you, but this can’t even compare,” said Kirk, the owner of an ice-delivery firm in Orlando, Florida. She said her health deteriorated in the years it took to resolve the dispute. “It felt like: ‘Let’s wear everybody down and see how much money we can glean from those that stop fighting back.”’

Kirk and other small-business owners signed up for FleetCor’s fuel cards hoping to organize their spending and save money at the pump. But the cards came with more than a dozen fees, including an account administration charge (10 cents a gallon or $2 per transaction), $50 to pay bills by wire, $3.50 per billing cycle if the cards aren’t used, and, for some, a surcharge if gasoline falls below $3.25 a gallon. The last time the price was that high was 2014.

FleetCor’s practices have made it a Wall Street darling. Since going public in 2010, revenue has climbed fivefold to $2.2 billion, its stock has surged more than 780 percent and Chief Executive Officer Ron Clarke has become one of America’s highest-paid bosses. He’s taken home $357 million since the IPO, an amount nearly equal to what Visa Inc. and Mastercard Inc., the world’s biggest payments networks, paid their leaders combined over the same time period.

Consumer advocates and some former FleetCor executives say the firm is an embodiment of bare-knuckles, winner-takes-all capitalism, and takes advantage of smaller fish who have little recourse but to complain to regulators or undertake costly litigation. Defenders say its hard-charging style has generated billions for investors and made it the largest U.S. fuel-card provider.

“The company figures out the easiest way to make money,” said Jeff Lamb, a former FleetCor executive. “That’s not a dumb thing to do — that’s smart.”

“A good company should be one that makes all its stakeholders happy — its investors and its customers,” said Bart Naylor, a financial-policy advocate at Public Citizen. “Just because a practice is legal, it shouldn’t be an invitation for exploitation.”

Regulators haven’t accused Norcross, Georgia-based Fleetcor of wrongdoing. The company said it believes its fees and billing practices comply with all laws and regulations.

“The majority of our more than 800,000 business customers are very satisfied with the value we provide, have no minimum contract period, and consistently choose to continue using FleetCor’s products and services, as evidenced by our sustained high retention rates,” the company said in a statement. FleetCor said in October its customer retention rate was 92 percent.

Since 2014, FleetCor customers have filed 366 complaints with the Federal Trade Commission. By comparison, Wex Inc., FleetCor’s top U.S. competitor and less than half its size, drew 44. In 2017, the grievances drew the attention of short sellers, led by Citron Research’s Andrew Left, who called FleetCor’s practices predatory.

“This is a business model intended to deceive,” Left told Bloomberg at the time.

Clarke, 63, shot back that the accusations were “fake news.” Shares ended the year up 36 percent. Wall Street analysts remain bullish on FleetCor stock, saying it should trade close to $237 in the next 12 months, a 17 percent increase from its current level. Clarke declined to be interviewed.

FleetCor’s customers are usually smaller companies with fleets of vehicles, such as trucking businesses, construction firms and shuttle services. The fuel cards give users a discount per gallon and help gas retailers attract more customers. FleetCor collects a slice of each transaction.

Clarke, who was recruited to the company in 2000 after stints at General Electric Co. and Automatic Data Processing Inc., has also expanded into corporate payments, gift cards, lodging and tolls. FleetCor, with an $18 billion market value, does business in more than 50 countries.

Fuel cards aren’t subject to the same regulations as consumer credit cards, which are governed by legislation meant to protect consumers from practices that include short payment deadlines and excessive fees.

Among the FTC complaints are scores of FleetCor customers who said online payments took days to register, resulting in late fees, and calls to customer service that meant hours spent on hold or empty promises of refunds.

“Every time you call them, it’s like you’re calling another planet,” said Arthur Bicherov, co-owner of Grizzly Movers, a family-owned company in San Diego that FleetCor sued for not paying bills. “People would hang up on me or transfer me to nowhere.” Grizzly and FleetCor have settled the legal complaint, he said.

A 2016 agreement for FleetCor’s Fuelman card lists 27 different fees. More recent terms and conditions posted on the company’s website show 15.

One customer told the FTC in 2017 that FleetCor in less than two years charged $8,438.58 in fees triggered by low gasoline prices.

“They also want you to go ahead and pay the fees, and say they will refund charges later,” wrote the customer, whose identity wasn’t revealed. “I have not seen a refund to date.

Some former FleetCor employees, including Lamb, say criticism about FTC complaints is overblown.

“Acting on customer feedback, both positive and negative, is a critical component of our success and we take customer complaints seriously in order to improve our products and services,” FleetCor said in the statement.

Customers are usually free to cancel their accounts at any time, but doing so can be disruptive and costly for small firms. Several business owners said they unknowingly paid fees for months because they didn’t read their bills closely.

At a 2016 conference, Chief Financial Officer Eric Dey said FleetCor focuses on small- and medium-size businesses in part because they “have very little pricing leverage.”

The strategy has fueled Clarke’s pay packages, which for years have bothered investors, with one accusing him of using FleetCor as his “personal ATM.” The board said last year it will pare Clarke’s compensation.

“We’re proud here of the way that we compete and the way that we play,” Clarke told analysts in 2017. “So don’t bet against us.”

Some customers have bested FleetCor. After a battle lasting more than two years, Kirk received a full refund. But others, like Larry Gazdick, simply stopped paying bills after FleetCor charged fees he said were unwarranted.

FleetCor sued Gazdick’s Newtown, Pennsylvania-based AV Rental Solutions but quickly settled for a fraction of the bill, he said.

“I equate it with the lowest of the low,” Gazdick said of FleetCor’s business practices. “Anyone who’s willing to immediately settle for pennies on the dollar kind of shows they were screwing with you to begin with.”