It might not be a good idea to overstuff your backpack next time you fly Frontier.

People don’t fly budget airlines like Frontier or Spirit because they’re known for their high quality customer service, or because they’re renowned for their roomy seats. While opinions will always vary, and people tend to only be inclined to leave a review when they are irate, low cost airlines are generally not known for their comfort.

Instead, people take these flights for their rock bottom prices, which can make up for all manner of inconveniences.

Frontier  (FRON)  has been through a lot lately, as it lost out on its plans to merge with Spirit Airlines  (SAVE) – Get Spirit Airlines Inc. Report, which will instead partner with JetBlue  (JBLU) – Get JetBlue Airways Corporation Report. In response, Frontier has made some that surely must please costumes, such as drastically increasing the amount of transatlantic flights it offers, including to Bahamas, Costa Rica and Jamaica, at prices starting at $69. 

The airline has also introduced an offer, which is good until the end of the year, that members of Frontier’s Miles loyalty program who purchase two round-trip tickets are eligible for a bonus of 20,000 miles for a maximum of 100,000 bonus miles.

But now the company has made another change, which might not be so warmly received.

Frontier Won’t Let You Get Away With Stuffing Your Luggage

The Denver-based Frontier Airlines will sell tickets that can sometimes cost as little as $19. But the reason it can get away with that price is that the airline will then proceed to charge you, often quite exorbitantly, for literally everything else.

A seat with extra legroom can run up to $25, and it can cost up to $99 to change or cancel a reservation, according to Sky Scanner. Even a simple can of soda or a bag of pretzels, which are usually included with ticket price, will cost a few extra dollars, as will confirming your reservation or selecting your seat.

But the main way that Frontier will get you is its baggage fees, which can often be even more expensive than your plane ticket. The Points Guy notes that “out-of-pocket costs for customers quickly go up when everything from bringing a full-size carry-on bag for $50 to $89 to traveling with a checked bag for $55 to $96 is factored in.”

But the one small (literally) perk that Frontier offers is that you can bring one personal carry-on item on board. It must be no larger than 14 inches by 18 inches by 8 inches, and it must fit under the seat in front of you. This is generally just a bit too large for most common backpacks.

If your item is too large, you can pay for one-carry on item, but “it cannot exceed 10 inches by 16 inches by 24 inches — including handles, wheels and straps — and must weigh less than 35 pounds.”

Many customers have long gambled that Frontier isn’t really checking these measurements that thoroughly, and will stuff their bags as much as possible to avoid having to pay for a bigger bag.

But now, Frontier has begun cracking down on this practice, as the company confirmed to The Points Guy that they are enforcing the rules, and that flight attendants are strongly encouraging passengers to step up to the counter to measure their large backpacks, roller bags and duffle bags, noting that they will be charged a higher price if they try to board with an unpaid bag.

How To Avoid High Fees

If you get caught trying to get away with a bigger bag, you risk getting hit with a fee, or not being allowed to board.

The cost of a carry-on bag is based on a variety of factors, including what your flight is and if you pay upfront for a bundle. But if you’re going to bring a bag (and most people need a change of clothes for their trip, unless you are going to a nudist resort), The Points Guy says your cheapest option is to pay upfront rather than trying to risk it. 

Additionally, though Frontier offers bundles, it’s best to carefully examine the options and to decide what you really need to pay for; if the bundle includes seat selection but that’s not important to you, it’s cheaper to just pay for what you want.