NEW YORK (CNNMoney.com) - More than 6 billion credit card offers were mailed last year...a record. And with gas prices also at record levels, the gas rebate cards are the hottest things around.

In today's 5 tips we'll tell you what you need to know before buying a gas rebate card.

Unlike other reward cards, like frequent flier credit cards, gas rebate cards have few stings attached and can offer significant savings, according to Curtis Arnold of the online credit card rating site cardrating.com.

If you have a credit score over 720 and you fill up your tank at least twice a month, gas rebate cards are a true value. Gas rebate cards can give you anywhere from 3 percent to 5 percent rebate on your purchase. So, if you spend $40 a week filling up your tank, you'll save about $2 a fill up. At the end of the year, that's over $100 in savings.

The rebate is either added as a credit to your statement, or a check you get in the mail. Some gas rebate cards may even give you an introductory period where you can save as much as 10 percent in the first two months.

There are two different types of gas-rebate credit cards. There are general credit cards that offer gas rebates as part of a larger reward program, and cards offered by specific oil companies that give you rebates for purchases at their particular gas stations.

Unless you have undying loyalty to a specific brand of gas, you're better off going with a card offered by credit card companies since you'll have more flexibility.

Beware that the average interest rates on these cards is about 14 percent to 23 percent, depending on your credit history. You'll reap the benefits of a gas card only if you can pay off your balance in full each month.

Otherwise, any balance you have left over will eat into your rebate. You also shouldn't have to pay an annual fee if you're shopping around for a gas rebate card.

While gas rebate cards can add up to significant savings, you'll still want to comb through the details for all the possible restrictions.

For one, make sure you get the limits on how much rebate you can get back. In most cases, that will be $300 to $600.

It's also likely you'll have to stay away from gas stations at warehouse clubs like Costco. Most of these cards require that in order to get your full rebate, you have to go to a standalone station that's a station who's primary function is selling gas.

And most times, in order to get your rebate you'll have to pay at the pump, not inside.

In some cases, you'll also have to be proactive. About 50 percent of the time, your rebate won't be automatically be deposited into your account. You'll have to contact your issuer in order to redeem your rebate. In some cases, you may only have 6 months before your rebate expires.

Last year, companies sent out about 14 million pieces of direct mail for oil and gas cards, according to Synovate, a market-research firm. As a customer, you are highly sought after.

"The credit card market is saturated, and in order to compete, issuers are trying to pull the carrot out on these gas rebate cards," says Arnold. And if gas prices go higher, you can bet that gas card rebates will become even more attractive.

So if you're looking for a gas card, make sure you don't depend entirely on the solicitations you get in the mail. There are about 40 thousand card offers out there, so you're only getting a slice of the pie. Go online and shop around.

Check out www.cardratings.com or www.creditcardguide.com to get more information.

And of course, don't rule out calling the issuer. Negotiation works. Remember, it costs credit card companies $100 to $150 in marketing efforts to get new customers, according to Curtis. So, take advantage of the competition. Chances are they won't be able to give you a larger rebate, but you may be able to get other concessions, like a lower interest rate.