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    Better check what's in your bag before you can take off.

When it comes to baggage fees, what you don't know can hurt you. Next time you book a flight, look closely at the fees and what’s in your bag. You may save some money.

  • 1. The fees for a single suitcase could cost more than your ticket.

    A recent round-trip flight on United between San Francisco and Los Angeles was a pretty good bargain at $118, but if you pack too much, the deal disappears. The following costs are based on round-trip travel on the airline.

    The first checked-bag fee is $50. That’s not bad. But add in the overweight charge (50-70 lbs.) of $200 and your deal disappears. Suddenly your $118 ticket has ballooned to $368.

    Tip: Carry-on bags are free on most airlines, but remember they come with size restrictions.

  • 2. With billions to be made in bag fees, it's amazing any airline still offers free bags. But some do.

    In 2007, checking a single bag on American, Delta, United or US Airways cost passengers $0. Five years later, those same airlines raked in over $2.5 billion in bag fees. Yet JetBlue still offers one checked-bag for free, Southwest offers two, and all of the above carriers allow free carry-ons. The real question is, for how much longer?

  • 3. Even airlines that charge for carry-ons allow some bags for free.

    Allegiant, Spirit and in some cases Frontier charge for carry-ons, with prices that can soar to $100. But some small bags are still allowed for free. Spirit, for example, allows one personal item sized 16 x 14 x 12 inches. Such bags typically must fit under the seat in front of you, but a small backpack or sports-type bag might work.

    Tip: Some airlines count a purse as a personal item

  • 4. No more gaming the system with oversized carry-ons.

    It's hard watching someone waltz to the gate with a giant carry-on that is then checked for free, especially when you play by the rules and pay the fee. United agrees. The airline is now putting baggage-sizers in airports so passengers with carry-ons larger than 9 x 14 x 22 will have to pay the normal checked-bag fee.

    Tip: United's size limits include handles and wheels.

  • 5. Paying baggage fees means multiple charges on your credit card statement.

    It would be nice if the airline charged you once for the combined ticket and bag charges, but that would be too simple. If you're puzzled by multiple charges on a statement, take a closer look: One is for airfare, the other the bag. It doesn’t mean you pay more. It just means your statement can be confusing.

    Tip: Airlines may not accept cash as payment for bag fees.

  • 6. If a bag is lost, the fee will be refunded. But bags are rarely gone for good.

    According to the Department of Transportation, bag fees must be refunded if bags are lost. Plus you are entitled to "reasonable reimbursement for expenses you incur while waiting for the delayed bag, such as the purchase of toiletries and a change of underwear." But bags are rarely gone for good. In November 2013, for example, the rate of “mishandled baggage” was just 2.62 bags per 1,000 passengers. About 98 percent of wandering suitcases are eventually reunited with their owners.

  • 7. Canceled or delayed flights likely mean a bag fee refund, but it may not be automatic.

    If your flight is canceled due to bad weather, do not assume the bag fee charge will be automatically removed from your credit card bill. American Airlines, for example, says you must request a refund.

    Tip: Hang on to your bag fee receipt; you will need it to claim a refund. 

  • 8. A baggage fee is not an insurance policy.

    The fee you pay to check a bag doesn't guarantee much beyond basic transportation, and it sure doesn't mean it'll slide down the carousel in a timely fashion. One minor exception: Alaska Airlines has a 20 minute guarantee that says if your bag doesn't make it to the carousel within 20 minutes of the plane parking at the gate, you get a $25 voucher good for future flights.

    Tip: As with most guarantees, there are exceptions, so read the fine print.

Rick Seaney is an airline travel expert and the co-founder of FareCompare.com, an airfare comparison shopping site