So you waited until Sunday night to do your tax return and realized you owe Uncle Sam a huge chunk.

Whether you file your tax return or request an extension, you've got to pay in full by midnight tonight. So what do you do?

If you have the money, make your check out to the "

United States Treasury

." Be sure to include the following information on your check: your Social Security number, a daytime phone number, the tax year and the type of form filed, e.g. 1999 Form 1040. Print out

Form 1040-V

-- Payment Voucher

and include it with your payment. Just don't staple the payment to your tax return. The

Internal Revenue Service

hates staples.

But what if you don't have the cash?

Pull Out the Plastic

Call 888-2PAY-TAX and charge your tax payment, extension payment or first-quarter 2000 estimated tax payment on your

American Express

,

Discover

or

MasterCard

.

As of March 31, 16,700 people charged their bills. Average hit: $1,362.

"And there have already been numerous payments over $1 million, with the largest so far over $5 million," says Bruce Zanca, senior vice president of

Official Payments

(OPAY)

of San Ramon, Calif., the company running the credit card payment system for the IRS.

If you charge your extension bill, the IRS won't require you to file

Form 4868

-- Application for Automatic Extension of Time to File U.S. Individual Income Tax Return

. Official Payments will accept your extension charge until midnight tonight.

Your first-quarter 2000 estimated tax payment is also due at midnight on April 17. And, again, if you charge the amount due, you won't have to file

Form 1040-ES

-- Estimated Tax for Individuals

.

Regardless of the purpose of your charge, you'll pay a "convenience" fee for using a credit card. The fee averages 3% to 4% of your payment. Check out Official Payments'

Web site for the complete payment scale.

Remember, if you file electronically using

Intuit's

TurboTax

or

MacInTax

software programs or Web sites, you can charge your tax bill using your Discover card only. (See this previous

story for more on credit card payments.)

Here are a few more charging tips before you dial 888-2PAY-TAX:

You can split your payment between two credit cards if your tax bill exceeds your credit limit on one card.

If you're charging an amount drastically higher than what you normally spend, contact your credit card company first so it allows the payment.

Make sure you have your Social Security number and credit card number handy. "Then the process will take you five minutes or less," says Zanca.

You'll get a confirmation number once your payment is accepted. Keep it with your tax records.

IRS Payment Plan

If your bill is $25,000 or less and you're willing to pay within five years, you can apply for the IRS installment plan. Attach

Form 9465

-- Installment Agreement Request

to the front of your tax return, listing your proposed monthly payment and due dates.

The IRS will respond to your request within 30 days. If your bill is $10,000 or less, you'll be automatically approved. If you're not approved, you'll owe the full balance within 10 days of receiving notification that the IRS has rejected your terms. "Explore other payment options in those 30 days to be safe," says Kathy Burlison, an

H&R Block

senior tax specialist.

When you apply for the installment plan, you'll be hit with a $43 application fee and interest on the amount due will compound daily at an annual rate of 9%. In addition, you'll owe a monthly late payment penalty of 0.5% of the balance. This rate drops to 0.25% when the IRS approves the agreement.

If you can't pay today but are expecting a chunk of money in the next few months, pay what you can now but don't ask for the installment plan, suggests Burlison. You'll have 120 days before the IRS takes serious action, like going after your wages or your property. You'll be hit with interest and penalties, but you'll avoid a $43 application fee.

Even if you're waiting to hear from the IRS about your proposed payment plan, keep in mind you still need to pay your other tax bills, including your 2000 estimated tax payments.

Why Extend?

If you can't get your tax return together by midnight you can file for a four-month extension. Your

Form 4868

-- Application for Automatic Extension of Time to File U.S. Individual Income Tax Return

must be postmarked April 17.

This extension buys you four months to get your return in, but any unpaid taxes will draw interest and penalties dating from midnight April 17 until you file your return. (See a previous

Tax Forum for more details on the extension process.)

While

TurboTax,

TaxCut,

hrblock.com and

TaxAct require you to pay $9.95, download the extension form and send it by snail mail. Other

online tax preparation sites will let you electronically file your extension form. In most cases, the fee can be applied toward the cost of filing your return when you finally get around to it. See the chart below for details.

Don't forget to check the extension requirements for your state return. Some states require their own form, while others will accept the federal Form 4868.

Send your questions and comments to

taxforum@thestreet.com, and please include your full name.

TSC Tax Forum aims to provide general tax information. It cannot and does not attempt to provide individual tax advice. All readers are urged to consult with an accountant as needed about their individual circumstances.