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You open your credit card bill and discover a charge that isn’t yours. The solution might be as simple as a quick phone call or a short letter. Or, it could be a situation that requires more time.

Knowing whether it was a billing error or a case of credit card fraud or theft can make a difference in how you handle the dispute.

Howard S. Dvorkin, a prominent debt and credit expert and author of “Credit Hell: How to Dig Out of Debt,” says, “People just need to understand the law and how it affects them. And don’t be afraid to dispute something.”

Short of consulting attorneys, going to court or complaining to financial regulators, here are seven steps you should take if you find an unfamiliar charge on your credit card bill.

1. Determine whether errant charges are due to theft or fraud

If it’s a case of unauthorized usage of your credit card, by law you’re liable only for the first $50 in unauthorized charges. However, most card companies cap your liability at zero dollars if you report the charges soon after discovering them.

If you suspect someone has hijacked your card or card number, call your card issuer and cancel the card and arrange for a replacement. The issuer may even be able to keep a closer eye on your account for a period of time.

Go through the most recent charges to your card with the issuer to make certain all of them are yours.

If you suspect theft or fraud, it’s a good idea to file a police report or a fraud affidavit with the Federal Trade Commission, says Chi Chi Wu, staff attorney for the National Consumer Law Center.

2. Store error? Try calling the retailer first

If you think the charge on your credit card bill was a retailer’s mistake and not account fraud or theft, you can try to straighten it out with the merchant, deal only with the card issuer, or a combination of both.

Retailers may accidentally double-bill you or charge the wrong amount for a purchase.

The law gives you 60 days from when you discover the billing error to notify your card issuer in writing, says Lauren Z. Bowne, a staff attorney for Consumers Union who works on issues related to credit cards and bank products.

That gives you some time, if you feel it might be easier to go through the merchant to fix the problem. It makes sense to contact the merchant first to see if it will take care of the mistake, and often it will. “The merchant wants to keep you happy,” Dvorkin says.

However, if the merchant doesn’t resolve your problem quickly, contact your card issuer within the 60-day window so that you preserve your dispute rights.

3. Write to the credit card issuer

You’ve called your card issuer about an unauthorized charge or merchant billing mistake. Or you’re still stuck with a billing error that the merchant failed to quickly correct. Now, it’s time to write your card issuer.

Summarize your previous attempt at resolving the issue, provide names and dates and state what it is you want the company to do. If you have proof the charge isn’t yours, include.

However, you’re not required to have proof to dispute a charge, says Ruth Susswein, deputy director of national priorities for Consumer Action, a nonprofit advocacy, education and outreach organization.

When you’re ready to send your letter, mail it certified with a return receipt requested. That will give you proof of the dates your letter was sent and received. Keep copies of letters you send, along with notes on the phone calls you made earlier.

Mail the letter to the address for billing disputes, not to the billing address, Susswein says.

The issuer must send written acknowledgement of your dispute within 30 days of receiving your letter.

Promptly complete any forms the issuer sends you and mail them back certified with a return receipt.

4. Know your rights and responsibilities

Once you raise a dispute with your card issuer, it must investigate and get back to you within two billing cycles – and no later than 90 days. You don’t have to pay a charge that’s being disputed, but you do have to keep paying the undisputed portion of your bill.

The card company can’t report you to the credit bureaus as paying late and they can’t charge interest on the disputed amount, Wu says.

Not paying the disputed amount preserves your rights to challenge the error under “claims and defenses,” Bowne says.

However, the card issuer can apply the disputed amount against your credit limit, according to the Federal Trade Commission. Disputing $3,000 in airline tickets on a card with a $10,000 credit limit would leave you with $7,000 in available credit until the dispute is resolved.

If the credit card dispute is resolved in your favor, the issuer should cancel the charge and related interest.

If the dispute is not resolved in your favor, you’ll be responsible for the charge and any interest that accumulated during the dispute period, Susswein says. If you don’t pay at that point, you risk hurting your credit, she says.

5. Be polite but persistent

Do you want to hold a winning hand throughout the credit card dispute process?

In phone conversations, stay calm and never raise your voice, even if the person on the other end of the phone does.

Do the same with letters you write. Be organized, factual and polite. Avoid speculation.

If it’s a legitimate claim, especially if it’s a high-dollar one, be persistent. And if your claim is rejected first time around, don’t take that as final.

6. Cite ‘claims and defenses’

If a merchant made a mistake on your bill and you didn’t catch it within 60 days of it appearing on your statement, you still have an option.

Even after the window for reporting a billing error to your card issuer lapses, you have the right to ask for a correction of a merchant billing mistake and withhold payment of the disputed amount by citing “claims and defenses,” Bowne says.

You have one year from the time of the disputed purchase to cite claims and defenses. To do this, you must not have paid the disputed amount, Bowne says.

Write a letter to the merchant, even if you’ve spoken by phone, outlining the mistake. Include copies of anything that proves your argument and ask the retailer to correct the problem.

7. Follow up with your card issuer

If you’ve asserted “claims and defenses” and the merchant refuses to budge, contact your card issuer.

“Give a call to get things rolling, and follow up with a letter,” Bowne says.

In your letter, succinctly outline the problem and desired solution. Detail exactly how you’ve tried to resolve the credit card dispute with the retailer and any responses you received.

Invoking “claims and defenses” requires you to have made a “good faith” effort to work it out with the merchant first, Bowne says.

Include any copies of evidence that backs up your claim. “Just try to keep as much of a paper trail as you can,” Bowne says.