NEW YORK (TheStreet) -- The deadline for open enrollment is today, and for people looking to buy health care insurance, the window for accommodating the individual mandate side of the Affordable Care Act has all but closed.

According to a study from the McKinsey Center for Health Care Reform, the uninsured make up just 27% of the total amount of new health care insurance signees through February.

As usual with uninsured Americans, the reasons for procrastinating come down to one issue: money. "The most common reason for not enrolling cited by both previously insured and previously uninsured respondents continues to be perceived affordability challenges," McKinsey reports. It was cited by half the respondents who had not yet enrolled.

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For health insurance shoppers looking to make good in the individual mandate, the watchword even at the deadline should be "caution." Rushing can lead to big mistakes, poor coverage and excessive health care costs, says Mountain View, Calif.-based eHealth.

"As we approach the end of the open enrollment period, there's still a lot of consumer confusion about the law, about when the enrollment deadline actually occurs and about the consequences of going without minimum essential coverage beyond March 31," says Gary Matalucci, a vice president at eHealth.

The company, which operates the nation's largest private health care exchange, offers a list of things to keep in mind when signing up:

Also see: Some Are Still Confused by Affordable Care Act>>

March 31 isn't just about a tax penalty. Consumers believe the mandate is all about avoiding a tax penalty, the company says, but that's not so. "There's more to it," eHealth says, pointing to the real issue: "March 31 is actually the last date on which many people will be able to get health insurance on their own for 2014. And while it was recently announced that people who have started applications but experience certain issues enrolling through or through the federal government call center by March 31 may be able to apply for health insurance after that date, this exception won't apply to everyone."

There is coverage outside government health care exchanges. Some Americans believe they can get coverage through private health care exchanges after March 31. Again, not so. "Unless you experience a qualifying event (such as -- but not limited to -- marriage, the birth of a child or the loss of employer-sponsored coverage) you may not be able to buy health insurance after March 31," eHealth says.

The tax penalty. Going without insurance does result in a tax penalty for individuals. But it's likely more than you think. "You might have heard that the annual tax penalty is only $95, but that's not quite accurate," eHealth says. "For 2014 the maximum tax penalty amount is $95 per adult and $47.50 per child within your household or 1% of your 'applicable' income. This means that depending on your income, your tax penalty could be hundreds of dollars per year. And penalties are scheduled to increase in future years."