You've completed your tax return and the only thing left to do is send in your payment. But even that step requires one more calculation: Figuring out which payment method is right for you.

No matter how you decide to pay the IRS, "the one thing you don't want to do is not pay and end up with a tax lien on your credit report," said Gerri Detweiler, a Sarasota, Fla.-based credit expert and author of the "Ultimate Credit Handbook."

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"That's extremely costly because it's very negative to your credit and it can trigger interest rate increases on your credit cards and difficult borrowing for many years until that is paid," she said.

If you're feeling crunched for money, it may help to know you're not alone: Even his high-net-worth clients on occasion are stretched thin when it comes to paying their tax bills, said Dan Yu, a certified financial planner and senior manager in the personal wealth advisers group at Eisner LLP, an accounting firm in New York.

"Interestingly enough, these issues do come up. We here at Eisner will negotiate on the client's behalf when there are real reasons why [taxes] weren't paid on time or they're disputing an item," he said. Those negotiations are often successful, he said, though the IRS is getting a bit less flexible in its willingness to work with taxpayers. Read more about negotiating with the IRS.

You can always try, said Francois Hechinger, a tax partner in the San Francisco office of BDO Seidman. "Call the IRS and let them know what's going on. Talk to an agent," he said. "If you call them, they're usually pretty willing to work with you."

IRS agreement or credit-card debt?

One payment option is to negotiate an installment agreement with the IRS. If you owe less than $25,000 and are able to pay your full bill over time, it's often possible to get the IRS to agree to an installment arrangement.

But you'll have to do the math to compare the cost of an IRS installment agreement with the cost of using your credit card. The IRS is currently charging an interest rate of about 8 percent (this rate fluctuates quarterly), and there's a possibility the IRS will charge a late-payment penalty as well (usually 0.5 percent per month), though taxpayers can sometimes negotiate to get this penalty waived.

If you have a credit card with a lower interest rate, it may make sense to use that instead, Detweiler said.

But, while you're comparing interest rates, don't forget to include the various fees. An IRS installment agreement costs $43, while the online tax-bill payment processors charge a 2.49 percent fee. On a $2,000 tax bill, that's $49.80.

There's another option: Mailing your payment via one of the checks your credit card company provides — again, compare the interest rate and the fee charged for using that check.

Keeping things friendly

You may want to use your credit card to avoid any negative interaction with the IRS. "If it's close, I would probably go with the credit card. Then you have a clean slate with the IRS," Yu said.

And, Detweiler said, on occasion the IRS files a federal tax lien even if you have an approved installment agreement in place. "The interest rate isn't bad and the fee's not terribly onerous, but what isn't always clear to consumers is, even if it's approved, [the IRS] may file a notice to secure a lien," she said.

"You may propose an installment agreement ... and end up with a tax lien on your credit report," she said. Before seeking an installment agreement, check with the IRS that it won't file a lien.

One more thing: If you decide to use your credit card, put that particular piece of plastic away after you pay your tax bill, Yu said. "Take that card and put it in a drawer," he said, because if you have an outstanding balance, "your next purchase is accruing interest on day one."

Plastic payment options

If you decide to pay your taxes with plastic, there are more options this year.

Link2Gov, the Nashville, Tenn.-based operator of Pay1040.com, earlier this year introduced IncomeTaxPayment.com, where taxpayers can pay their tax bill with a PIN-based debit card. At the company's other site, Pay1040.com, taxpayers can use a credit card to pay their taxes.

OfficialPayments.com, the other online tax-bill site and a subsidiary of Tier Technologies of Reston, Va., accepts credit cards and branded debit cards (i.e. those that work just like credit cards).

At Pay1040 and OfficialPayments, you'll pay a convenience fee totaling 2.49 percent of your tax bill. That is, if you owe the IRS $1,000, you'll pay a $24.90 fee to charge it, not including any interest charges that accrue.

Meanwhile, at IncomeTaxPayment.com, you'll pay a flat fee of $2.95 for the PIN-based debit-card transaction. Is it worth it to shell out that fee when you could just mail a check (if you're able to use your debit card that means you have the cash on hand to pay your bill)?

The answer is: It depends. If you normally mail your IRS check with just a postage stamp, that's the cheapest way to do it. But some tax preparers say it makes sense to send your tax bill via certified delivery, return receipt requested.

"I recommend you send it certified," Hechinger said. "That's the only proof you have that you did mail it on a timely basis."

Those two services combined cost $4.25, not including postage, according to USPS.gov, a bit more than the $2.95 for the online debit-card payment.

Short-term pain

If you're not able to pay your full tax bill now but know that it's a short-term money crunch, consider paying your state bill first, Hechinger said. "If it's short-term and you know it's short-term, I would choose the state first, especially in California. They're not as forgiving" as the IRS.

Another option: Delaying the pain through a short-term extension with the IRS. "Occasionally you can get a short-term extension of time to pay up to 120 days," Detweiler said.

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Still, she added, one tax attorney told her the IRS doesn't approve the majority of such requests, plus the form is rather onerous. "For most taxpayers, they're going to look for a different option, but it is something to check out if you're really in a crunch and you're expecting some money, a property to sell soon, for example."

See this IRS publication for more information on payment options (PDF).

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