7 airline fees that can turn a cheap flight into an expensive trip

A $45 checked-bag fee may not look like much when you're booking a flight. But pay it in both directions for a family of four, and that one fee becomes $360.

That's the math travelers increasingly have to do when comparing airfares. Airlines have spent the last couple of decades pulling pieces out of the ticket and selling them back one at a time, which means the fare you see in a flight search may be only the starting price. Add a checked bag or a seat next to your kid, and the fare you started with may no longer be the cheapest option.

Here are seven airline fees that can catch many travelers off guard, plus how to account for them before you book.

• Related: New 'basic business' fares promise big savings. Travel experts say the cheaper ticket isn't always the better value.

1. Checked bag fees that erase a fare advantage

Most domestic economy fares don't include a checked bag, so checking one means paying the fee in each direction.

American Airlines, for example, charges $50 at the airport for the first checked bag on many domestic Main Cabin itineraries, or $45 if you prepay online or in the app. Delta charges $45 each way. United charges $50 at the airport, or $45 when prepaid.

If you can't fit everything you need in a carry-on, certain airline credit cards can help waive the checked bag fee on qualifying flights.

Include bag fees when comparing fares:If none of your credit cards waive the baggage fee, use flight search platforms like Kayak and Google Flights, both of which let you filter results to include checked-bag or carry-on costs. This lets you compare fares on what you'll actually pay.

2. Basic fares that make you pay for what's missing

A checked bag fee is, at least, honest — you see the charge, and you know what it buys. Basic fares, on the other hand, show you a lower cost up front, while the real trade-offs are in everything the ticket leaves out.

What a basic fare restricts depends entirely on which airline sold it. United, Delta and JetBlue each draw the line in a different place, so the same "basic" label can mean different trade-offs depending on who you're flying.

• United Basic Economy:On domestic flights, you can bring one personal item, but not a full-size carry-on. If you get your Basic Economy boarding pass online and arrive at the gate with a carry-on, you'll have to pay $75 to check it. Also, ticket changes aren't allowed, so you generally have to cancel and rebook. If you cancel, you'll receive only a partial travel credit.

• Delta Main Basic:The basic fare ticket earns no miles or Medallion status credit. Changing or canceling costs $99 for trips within the U.S., Canada, Mexico, the Caribbean or Central America and $199 for other destinations. You'll also pay the fare difference, if there is one, when changing flights.

• JetBlue Main Base:Seat selection isn't included, and cancellations will get you a travel credit, minus any applicable fee. Just be sure to check the current rules before booking, since JetBlue is transitioning to its new fare structure.

That means the real cost of a basic fare can include lost flexibility, carry-on fees and miles you won't earn. Depending on the airline, you may also be giving up the certainty of a seat you chose next to your family or friends instead of one assigned at the gate.

Read the fare rules before you book:Check what each basic fare excludes, add the cost of anything you'd need to buy back and count that toward the ticket price.

3. Seat fees that multiply on a connecting flight

Seat selection can look cheap because airlines show the charge for one traveler and one flight segment at a time. But each leg of a connecting itinerary has its own seat assignment, which means a connection can double the number of seat fees you pay.

Frontier, for example, charges $15 or more to pick a seat for each passenger per flight segment. On a Frontier round trip with a connection, a family of four needs 16 seat assignments at a total cost of at least $240. So a Frontier fare that starts out $200 cheaper for the family than a fare with seat selection included ends up costing $40 more once you add seat selection.

• Related: Is there really a cheapest day of the week to fly? We asked airfare experts to settle the debate.

Several airlines skip the fee for young kids. On Alaska, American, Frontier, Hawaiian and JetBlue, a child 13 or younger sits next to an accompanying adult without an extra charge, according to the Department of Transportation. These guarantees come with conditions, including that the child and accompanying adult travel on the same reservation and that adjacent seats are available in the selected class when booking.

Price seats by flight segment:Count the outbound and return segments for every traveler while comparing fares and run that math before assuming the seemingly cheaper connecting itinerary is the better deal.

4. Pet fees that add hundreds to a round trip

A pet traveling in the cabin doesn't take up a seat, but airlines still charge a separate fee for the carrier. The fee generally applies in each direction, so a round trip can double the advertised charge.

Many major U.S. airlines charge roughly $100 to $200 each way for a pet in the cabin. For example, Delta charges a $150 cabin pet fee within the U.S., Canada, Puerto Rico and the U.S. Virgin Islands. Southwest adds a $125 fee for mainland U.S. travel. Alaska collects a fee of $100 to $200, depending on the type of travel and route.

That puts a round trip at $200 to $400 before any partner airlines charge their own pet fees. Alaska says a connecting flight on another airline within the same itinerary may charge that airline's own pet fee in addition to Alaska's.

And for pets flying in the baggage compartment, airlines typically won't transfer them directly to a connecting carrier. You'll need to claim your pet at the connection and recheck it with the next airline before continuing your trip. Keep in mind that some airlines may not accept pets in the baggage compartment due to summer heat or because a regional jet lacks a climate-controlled cargo hold.

Confirm your pet's travel:Make sure the airline accepts your pet on every segment, then reserve space and price both directions before deciding which fare makes the most sense.

5. Lap-infant fees that show up on international flights

You may think that infants who sit on your lap don't need their own ticket. But while lap infants can often fly free on domestic trips, international travel may require an additional fare.

Delta charges 10% of the adult fare for an international lap infant, while Alaska applies a similar 10% rate plus taxes on long-haul international flights.

How you add a lap infant varies by airline and itinerary. Delta lets travelers add one online, while Alaska requires a reservations agent for long-haul international lap-infant tickets. Skipping this step risks problems at check-in, since the airline won't have a record of the infant on your reservation. 

Get the infant total before buying the adults' tickets:Find out how much it will cost to include your infant, including taxes and fees. And keep in mind that 10% of a premium-cabin fare can run from several hundred dollars to more than $1,000.

6. Unaccompanied minor fees that stack per child, each way

Sending a child on a flight alone comes with a service fee that most airlines charge in each direction.

Southwest charges $100 each way per child for unaccompanied minors ages 5 to 11 for U.S. mainland travel. Delta charges $150 each way, but one fee covers up to four children. JetBlue charges $150 per child each way. Two siblings on a round trip can add $300 to $600, and potentially more on other airlines. 

However, the fee isn't the only consideration: Some itineraries don't allow unaccompanied minors at all. Southwest allows unaccompanied minors only on nonstop or same-plane flights and doesn't offer the service internationally. JetBlue requires every child ages 5 to 13 to use the paid service, with no way to decline it, while Delta enforces the same rule through age 14.

That means the cheapest itinerary may not even be an option, regardless of the service fee. A connection, an international route or a child's exact age can change which airline will accept the trip.

Review the unaccompanied minor rules before paying for the fare:Enter the traveler's age, confirm the route is allowed and check whether siblings share one fee.

7. Agent assistance fees that count per traveler

Calling the airline to get help with your reservation feels like one transaction, but some carriers price it per person on the same reservation.

JetBlue charges $25 per person for a new booking, change or cancellation completed by phone or chat, so one call for a family of four adds $100. However, JetBlue may waive the fee in certain situations, including for lap infants and customers who can't complete the task online.

Frontier applies the same idea at the airport: agent help with tasks like checking in or printing a boarding pass costs up to $25 per passenger, per direction, so a family of four could pay $200 round trip. Frontier exempts disability-related assistance, active-duty military travelers and customers adding a lap infant or pet.

Check the self-service option first:Try the app or website before calling or heading to the counter, and ask whether a live agent request carries a fee before you agree to it.

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What airlines may owe you back

Airlines call many extras nonrefundable, and that's usually true when you simply change your mind. However, it's a different story if the airline took your money and never delivered the service.

• Delayed checked bags:Your checked-bag fee is refundable when the bag is significantly delayed — more than 12 hours after arrival on a domestic flight, 15 hours after an international flight of 12 hours or less, or 30 hours after a longer international flight.

• Services you didn't receive:A paid seat, Wi-Fi, lounge access or other extras you paid for should be refunded if they were unavailable through no fault of yours. A "nonrefundable" label doesn't let an airline keep your money for a service it failed to provide.

• Canceled or significantly changed flights:If you don't accept the airline's alternative transportation, you're entitled to an automatic refund to your original payment method. For credit card purchases, the refund is due within seven business days.

• The 24-hour rule:For a ticket bought at least seven days before departure, the airline must either let you cancel for a full refund within 24 hours of booking or hold the fare without payment for 24 hours, though it doesn't have to offer both. This rule doesn't apply to tickets bought through an online travel agency or travel agent, one more reason to book directly.

If you do face an issue, keep all confirmations, fee receipts, screenshots and baggage report numbers until you receive a satisfactory resolution. The Department of Transportation also publishes what each airline has specifically promised during a disruption within its control, including rebooking, meals and hotels. You can look up your airline's commitments if you're stuck at the gate and hold them to it.