If you needed a clue that Chicago's Eye Spy Optical does brisk business in late December - the time when many healthcare flexible spending accounts expire for American employees - just take a look at its storefront in the city's Lakeview neighborhood.

You'll see three signs alerting passersby to spend those flex dollars, including a sandwich board that warns: Use it or lose it. Only - days until Flex Spending Account Deadline!!

It definitely gets busy, says Eye Spy owner Alissa Fields, who stocks chic frames mostly in the $250 to $550 price range. And while it might take a few weeks to fill an order with prescription lenses, Eye Spy customers walk out with purchase receipts in minutes, which is all the paperwork they need to spend remaining flex dollars.

Hence, no elaborate New Year's plans for Fields and her husband. On New Year's Eve, it gets insane, she says. One year I stayed here three hours after closing, and I couldn't go out afterwards, I was so exhausted.

Flexible Spending Accounts, or FSAs, are voluntary programs employers use that let workers set money aside annually through payroll withholdings. The money doesn't get taxed and can go toward certain medical expenses not covered by insurance. Yet a surprising number of workers don't take part: Mercer's 2011 National Survey of Employer-Sponsored Health Plans shows that healthcare spending accounts are offered by 85 percent of employers, yet have an average employee participation rate of just 22 percent.

That alarms some observers, given that FSAs can translate into big savings for many workers. Depending on your tax bracket, you might save anywhere from $250 to $350 per $1,000 in a flex account. (Many employers cap maximum contributions at $5,000 or lower.)

But cost savings only occur if you use up your money, so that what's unspent can cancel out the tax advantage. Mercer's survey shows the average annual contribution at $1,490, with 4 percent of those dollars forfeited. Based on a $2,000 contribution, that equals an $80 forfeiture.

Joan Vines, tax senior director at BDO US in Washington, D.C., says unspent flex dollars average out to about $100 per person in a flex plan. The money either goes back to employees or is used by companies to defray healthcare costs.

Employers have the option under Internal Revenue Service guidelines to give employees until March 15 to spend flex dollars for plans ending December 31. If the plan has a different end date, it's two and half months from then.

But the grace period isn't automatic. So ask yourself, do you have a few days to get rid of the money, or two and a half months?

THE FINE PRINT

Linked to that question is an all-important consideration: What can you spend the money on? Until recently, flex dollars could purchase over-the-counter medications, but that no longer applies, says Steve Wojcik, vice president of public policy for the National Business Group on Health in Washington, D.C.

You can't just run out and get Tylenol or antacids the way you used to do, Wojcik says. That's a big change from last year, and part of the federal healthcare law. The law also produced another change taking effect in 2013, when annual FSA contributions will be limited to $2,500.

The details of what you can spend on are outlined in plain English - sort of - in I.R.S. publication 502, Medical and Dental Expenses: For use in preparing 2010 Returns. The 34-page document covers categories from acupuncture to X-rays. Wigs are eligible for deductions (if purchased on physician's advice), but dancing lessons are not (even if a fleet-footed doctor suggests them for physical or mental health).

Meanwhile, it might baffle some taxpayers to learn that you can use flex money for Bactine and other first-aid items - and you can use it for sunscreen, so you may want to stock up for next year, Wojcik says.

As a result, many folks use flex spending deadlines to replenish family first-aid kits. Other items on the federal government's approved list include prescription refills, pregnancy test kits, eye surgery to treat defective vision and in vitro fertilization.

Yet, for those sweating the deadline, eyeglasses are the classic standby because a) they burn up flex dollars quickly and b) they don't require extensive medical procedures or appointments in advance. As Fields notes, It's not like you're going to get some sort of elective surgery at the last minute.

A nice pair or prescription shades with sporty frames or contact lenses make for an easy score, though.

Knowing this, Fields is bracing herself for an onslaught of customers come New Year's Eve. And that caricature of a smiling dollar bill flexing its muscles on her signage could apply to her year-end profits as well.

I feel like the last day of the year is like Black Friday, she says. Or Freaky Flexday, maybe.

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The author is a Reuters contributor. The opinions expressed are his own.

(Editing by Lauren Young and Beth Gladstone)