This post contains references to products from one or more of our advertisers. We may receive compensation when you click on links to those products. The content on this page is accurate as of the posting date; however, some of the offers mentioned may have expired. For more information check out our Advertising Disclosure. Budget airlines like Spirit, Frontier, and Allegiant are notorious for their low fares in exchange for high fees on baggage, seat selection, and well, almost everything. But if you're flying one of these ultra-low-cost carriers, you might be paying a fee of $40 or more per ticket without even knowing it.

Spirit tacks on a “Passenger Usage Charge” of up to $23 per segment. Allegiant levies a $22 “Carrier Usage Charge.” More recently, Minnesota's Sun Country joined the ranks by charging a $22 (or more) Passenger Interface Charge. But they're all the same thing: A fee for booking flights online.

Yes, really: In the 21st century, budget airlines are charging passengers more for the convenience of booking their flights online whether you book directly with the airline or through a third-party site like Expedia or by phone. These additional booking fees are shockingly common though still little-known, even among fairly frequent flyers.

Budget airlines are counting on it escaping travelers' attention and they're hoping you don't realize there's a fairly easy way to save that $40 per ticket or more in fees by booking your flights at the airport instead and not just for a flight that day, but for a trip weeks or months in advance. For a family of four, that's more than $150 in savings.

Here's how and why this loophole exists in the first place.

Why This Works No major U.S. carriers like American, Delta, United, or even Southwest levy a similar charge for doing so. But in the world of budget airlines which make their real money charging passengers for extras like seat assignment, bags, and even water onboard this fee is widespread.

From the big budget carriers like Spirit and Frontier to newcomers like Breeze, every low-cost carrier calls it something different. But they're all essentially identical: A fee for booking online or by phone. And given many of these airlines sell flights for $40 each way or less, that fee can eat up a majority of what passengers are actually paying for their flights.

I know what you're thinking: Can you only benefit from this money-saving method by purchasing a ticket for a flight you'll board that day?Not at all:You can book a ticket in person at the airport for weeks, even months in advance and still save that $20-plus each way.

Take, for example, this one-way Spirit ticket from Minneapolis-St. Paul (MSP) to Las Vegas (LAS) for under $39. The fare itself is less than $1, while various government taxes add a few bucks more. But the airline's Passenger Usage Charge of nearly $23 each way comprises nearly 60% of the ticket cost!

Why don't these carriers just raise their actual fares? Airlines are on the hook to pay a 7.5% federal excise tax on their fares… but not for these separate fees.

The same is true for extras like seats, bags, and more. Charging these fees separately rather than simply building them into the actual airfare means low-cost carriers can drastically reduce how much of their revenue they need to fork over to the federal government. That's a big part of the reason why all these add-ons are such a core part of budget airlines' business models.

But there's a hitch. To consider it a fee instead of airfare, airlines have to make it optional it's up to you whether you bring a bag, right? There's just one way to avoid these online booking fees: The airport workaround.

Buying your tickets with low-cost carriers in person at the airport means you can bypass these fees altogether. Is it worth a trip to the airport to save $40 or more on a ticket? That depends on how many tickets you're buying and what it takes for you to get to the airport.

But first, let's look at the U.S. airlines that tack on this pesky fee.

Airlines That Charge Online Booking Fees Scan the list, and you'll see that almost every budget airline in the country big and small, new and old tacks on an additional booking fee.

Exactly what it's called and how much it costs varies from airline to airline. The fees have increased over time, too: What used to be a surcharge of just $13 or so each way just a few years ago has increased to at least $20 or more on most budget carriers. In some cases, it adds $50 or more each way to the cost of your ticket.

No matter what airline you book, you'll be paying these fees on budget carriers whether you:

Book online directly through the airline's website Book online through a third-party online travel agency like Expedia, Hopper, and many more Book by phone Some airlines tack on a flat fee no matter what flights you're booking, while the online booking penalty varies for others.

Allegiant Air Allegiant levies what it calls an Electronic Carrier Usage Charge on every ticket. It's a flat $22 per passenger, per segment fee.

That means it adds $44 on each and every Allegiant roundtrip ticket when you book online or by phone. Head to the airport instead, and you could book this $80 roundtrip fare from Baltimore (BWI) to Sarasota (SRQ) for just $36 total.

Breeze Airways Breeze is another relatively new low-cost carrier, operating flights between smaller cities up and down the East Coast and Deep South. And its online booking fee is among the worst.

Breeze lists a Technology Development Charge on its website, but doesn't list how much that fee is. In practice, it ranges from under $20 to $40 or more per person, each way. Some passengers have noticed fees as large as $54 each way!

On this $69 one-way flight from Akron (CAK) to Fort Myers (RSW), for example, the airline is tacking on a $33 charge for booking online. That's almost half the cost of the ticket!

But Breeze also makes it harder to avoid this fee by booking flights at the airport than any other airline. The airline says it will only sell tickets in-person during a two-hour window once a week: on Tuesdays, from 11 a.m. to 1 p.m. ET, as of publication.

Want to book several Breeze tickets? The airline says that, in fairness to all guests, you'll have to go to the back of the line in order to book a second trip.

Frontier Airlines Frontier levies what it calls a CIC, or Carrier Interface Charge. As of publication, Frontier says it charges up to $23 per passenger, per segment.

That's what you'll almost always see when booking online with Frontier on all but the cheapest tickets. So while there's a $23 charge on this roughly $39 one-way fare from Minneapolis-St. Paul (MSP) to Denver (DEN)

it's just $4 when that same route drops below $19 each way.

Spirit Airlines Spirit is the airline most Americans think of when the words low-cost carrier come up. So of course, Spirit charges this fee, too.

With Spirit, it's called a Passenger Usage Charge. Exactly how much that costs you when booking online varies: It ranges from just $3.99 up to $22.99 per segment, as of publication. The cheaper your total fare, the lower you can (generally) expect these fees to add.

See our Spirit Airlines Baggage Fees guide to learn how to save even more when flying Spirit.

Sun Country Airlines Sun Country quietly joined the ranks of airlines tacking on these sneaky fees back a few years ago. It took customers a while to notice.

As of publication, Sun Country adds a “passenger interface charge”of at least $22 per segment, per passenger –an extra $44 or more on every roundtrip ticket.

And that's not a flat, $22 each way fee it could be even more. Sun Country's terms clearly state they charge “from $22 per passenger, per segment,” leaving the door open to an even steeper surcharge. That said, it reliably seems to be $22 each way (or $44 roundtrip) no matter where you fly unless you make a connection, in which case it doubles.

Back when it first started tacking this fee onto your tickets, Sun Country never clearly displayed how much it was adding in these extra fees and government taxes it merely said the final price includes all applicable air taxes, fees, and carrier-imposed charges.

So when we broke the news about this sneaky charge, we said that lack of explicit disclosure during the checkout process made it among the worst on this list: Sun Country customers were likely completely unaware they were paying it when booking online. Case in point, it escaped travelers' notice for more than a year!

Fortunately, that has changed. You can now click to see the breakdown of what's getting added to your ticket price before checking out with Sun Country.

Read more:Sun Country Now Adds a Sneaky Fee of $44 (or More) But You Can Avoid It

Want to save an extra $44 or more on your next Sun Country flight? Your best bet is to make your way to the airline's check-in desks in Minneapolis-St. Paul (MSP), which are typically staffed throughout the day. The airline will sell you a ticket in person at the Expedite Service counter and you can even add in bags and seat assignment (for an extra fee just as you would when booking online) while you purchase on the spot.

Book Your Tickets at the Airport to Save Saving $20 or more on your (already cheap) budget airline ticket sounds awesome, right? Well, here's the catch: You have to go to the airport to avoid this fee.

For some, this will be easy and well worth the trouble especially if you're booking flights for the whole family, live close to the airport, or can simply buy more tickets while you're already at the airport for a flight. For others, it would add too much in gas and parking or Uber fees for it to make sense.

Just keep in mind: You're buying your ticket in advance and then returning home not buying tickets the day of your flight.

Here's how to prepare for your budget airline ticket booking at the airport:

Know exactly which flight you want to book (with a few backup options) before you go. Use Google Flights to find the cheapest flights that work, determine how much you can save by bypassing these fees, and then head to the airport to buy them. If you're going to the airport soon for a trip anyway, plan to leave some extra time to purchase a future flight at the desk before going through security or before heading home. Make sure your airline is open for ticketing and that could be a major challenge with some carriers. Spirit's preferred ticketing hours are fairly generous at many airports, while Allegiant's airport ticketing hours leave just an hour or two once a week at some locations. Breeze will only sell tickets in person on Tuesdays from 11 a.m. to 1 p.m. ET, while a Sun Country spokeswoman said the airline can sell tickets at its Minneapolis hub during normal operating hours. Here's Frontier's airport ticketing information. Have a plan for the most affordable way to get to the airport. Whether that's taking public transit, parking at an affordable ramp, or having a friend drop you off and pick you up, that's up to you! Just calculate whether the cost of driving to and/or parking at the airport is worth the money you'll save. Unfortunately, not every budget airline ticket agent will play so nice and some may have never sold a ticket before. Prepare to be persistent to ensure you get the entire fee waived. These savings may come at a slight cost: steeper baggage fees. Many budget airlines like Spirit offer the lowest prices for carry-on bags or checked luggage when you buy them during the checkout process online and that's not always an option when you go to purchase at the airport. Give it a shot, but keep in mind you might be on the hook for slightly higher baggage and other fees by modifying your ticket online afterward. With your fares picked out and a plan in place, head to the airport for your fee-free budget airline ticket.

Bottom Line Is this little-known trick worth the savings? That depends on how much it saves you per ticket, how many tickets you need to buy, and what it costs you (in money and time) to get to the airport to avoid paying these pesky fees.

In the end, only you can decide.But this is a potentially easy way to make your cheap flights even cheaper.