With just three days left to file 2014 taxes, tax preparer Mary Arthur and her colleagues at the Campaign for Working Families at 31 sites across Philadelphia and South Jersey are hard at work.

But even as the tax season winds down, Arthur, the group's executive director, already is fretting about next year.

She is worried because statistics from early March show that of the 10,000 tax returns filed, 2,800 were from people who didn't have health insurance last year and were paying a penalty. Often, it's higher than the $95 minimum fine.

Of equal concern: some clients who had insurance last year but underestimated their income, meaning they must repay the government part of the subsidy they received.

"I'm seeing some people owing this year," said Arthur, whose organization prepared 18,000 returns last year. "It's a lot of money, and I'm afraid for next year."

In January, the Treasury Department estimated three million to six million Americans would pay a penalty for not having health insurance last year. The penalty for 2014 is at least $95 for an adult and $47.50 for each child under 18, ranging to a maximum of $285 per family, or 1 percent of household income, whichever is greater.

In 2015, the fines shoot to $325 for an adult and $162.50 for each child, or 2 percent of household income.

The U.S. Department of Health and Human Services has extended enrollment on the federal marketplace until April 30, giving people who didn't buy insurance one last opportunity to get covered and avoid being penalized for 2015.

Buying coverage during extended open enrollment will not absolve consumers from paying last year's penalty. After April 30, consumers can buy insurance from the marketplace only if they experience a life-changing event, such as marriage, a baby, or loss of job-based health insurance.

Arthur said most of her clients had heard about the penalty but did not "really realize what that meant."

According to the Kaiser Family Foundation, a single person making $19,650 and a family earning $29,800 would pay the $95 penalty. Everyone above those levels will pay more, and sometimes substantially more.

Arthur prepared a return for a woman whose household included three uninsured adults - the woman and two adult sons, whom she claimed as tax exemptions. The woman was on track to get a $140 refund until the 1 percent penalty was applied to the household income. Instead of receiving a refund, the woman owed the government $600.

"If you have children [over 18 and not in school] still living at home and you still carry them as an exemption and they don't have insurance, then the penalty comes out of your pocket," Arthur said. To many people, "that is a shocker."

Arthur recommended the woman buy insurance and referred her to an Enroll America navigator. The two groups are working together to help people buy health insurance during the extended open enrollment. At last count, 800 people had had an appointment with a navigator.

"We have navigators placed in our tax sites so people have the opportunity before the deadline to get enrolled," Arthur said. "It is worth it to have medical insurance. You are going to pay one way or the other, so why not pay and get covered?"

Arthur is also concerned that people are lowballing their income estimates to get a bigger subsidy. She tells her clients they must supply the marketplace with honest, accurate information.

"You have to say, 'This is what I'm going to earn,' " she said. "If you put in a lesser amount, at tax time you will have to pay that back."

A study by Kaiser found half of subsidy-eligible households would owe some repayment. The campaign's tax preparers suggest people reduce the chance of owing money by increasing paycheck deductions. Clients with pension incomes can do the same thing by asking their companies to increase their deductions.

"It's not whether or not you get a refund," Arthur said. "It's about not owing."

The tax-deductible silver lining to this year, Arthur said, will be the word-of-mouth campaign of friends telling friends how much they had to pay because they didn't buy health insurance.

"The people that are getting the penalty this year have had their eyes opened," she said. "They are going to tell their friends.''

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This article was written in partnership with Kaiser Health News, an editorially independent program of the Kaiser Family Foundation.