Thinking about slapping Uncle Sam's bill on your credit card? It might cost you a lot more than you think.

Worried about how you're going to pay your tax bill? It might be tempting to put it on your plastic. After all, you've probably heard you can charge taxes due on your 2000 federal income-tax return on your MasterCard, Discover Card or American Express. (But no Visa cards, please.) And what if you want a filing extension? No problem. You can just charge what you expect to owe the IRS. And if you owe estimated taxes for tax-year 2001, you can charge those, too. In fact, in some states, credit-card payments are available for your state income-tax bill as well.

Clearly, charging your taxes is convenient. And with the right card, you can even rack up some extra frequent-flier miles (or other goodies) to boot. So what's wrong with this picture?

The "convenience fee," that's what. It amounts to a hefty 2.5% of the amount you charge. It's like the fees merchants pay credit-card companies when you charge your purchases. Only in this case, the "merchant" is the Internal Revenue Service, and Uncle Sam wasn't interested in turning 2.5% of his revenues over to the card companies. Until now, you may have been blissfully ignorant of these merchant fees, but you will become painfully aware of their bite when they come directly out of your own hide. The money is collected by one of the two vendors that facilitate these transactions (Official Payments Corp. and PhoneCharge), and split with the card issuers.

Granted, paying $10 for the convenience of charging a $400 tax bill to your MasterCard isn't really a sin. But what about paying $125 on a $5,000 tax bill? And in addition to that, your friendly credit-card company steps up and starts charging you interest (often at 18% or more), unless you pay off your bill within the grace period. Bottom line? Paying taxes with your credit card is a great deal for all involved, except you.

Our suggestion? Try to find other (less expensive) ways to raise the cash needed to pay your taxes. Perhaps your credit union, your parents or your rich brother-in-law. Also, don't overlook the IRS itself. You may qualify to set up an installment payment plan with the government. If so, this is often the cheapest way to go.

If you are insistent on considering the credit-card option, visit officialpayments.com or about1888alltaxx.com to process your payment. But please think twice before going this route.