“My jaw dropped when it was like, ‘And you’re going to have to pay $100 per bag,’” said the Tampa Bay law professor and author of the upcoming book “Corporatocracy.” Her reaction: “Well, screw it because once you add in the price of the seat and the price of your baggage, it’s the same high price as the price I was trying to avoid with like United and Delta and American.”

As Torres-Spelliscy discovered, adding a bag to your flight can dramatically increase the cost — and the cost can be a question mark until just before the airline asks for your credit card. While some companies have uniform bag fees, most embrace some sort of variation, whether it’s a discount of $5 for booking in advance or factors that only the carrier’s algorithm understands.

“It’s a nightmare,” said Kyle Potter, executive editor of the website Thrifty Traveler. “This is quite literally my full-time job and I’m constantly looking at our own stories that we’ve written about increases to bag fees. Is it still $30, but that one’s $40 but $35 if you pay online in advance?”

Figuring out how much your checked or carry-on luggage will cost on some airlines can seem like solving those algebra word problems you thought you’d never use: When are you flying? What’s the route? When will you pay? Enter it all in the calculator — or check an increasingly complicated section of the website — to find out.

On JetBlue, for example, bags cost more during peak travel times, and when purchased within 24 hours of a flight. Budget airlines including Spirit and Frontier have bag fee calculators — Spirit’s is the “Bag-O-Tron” — that spit out the cost for checked and carry-on bags after travelers enter their route and dates of travel. And even major airlines including American and United charge less when travelers pay online or in advance. Southwest remains an outlier, allowing a carry-on and two checked bags for free.

The fees are a far cry from the mid-2000s, when the biggest carriers didn’t even charge for the first checked bag. American introduced its fee in 2008 and others followed suit; most raised prices this year for the first time since 2018.

“How did we get here?” asked Jay Sorensen, president of IdeaWorksCompany, which consults on airline fees. He let out a heavy sigh before proffering his answer: “My industry loves to make things too complicated.”

His company estimates that bag fees generate more than $33 billion in revenue globally. In the United States, airlines collected more than $7 billion in bag fees last year, according to the Department of Transportation. Bag fees don’t get hit with the 7.5 percent federal tax that is charged on domestic airfare, making them more profitable for airlines.

Sorensen said airlines are looking to maximize the amount of money they’re bringing in — but also increase loyalty by providing discounts or free bags for people who have status with a particular airline or hold their branded credit card.

Travel analyst Henry Harteveldt, president of Atmosphere Research Group, said it was “inevitable” that airlines would vary their prices for bags as many have moved to a model of charging a low base fare and tacking on additional fees for the options that travelers want to use. He said some travelers could benefit from fees based on distance; rather than everyone paying a uniform price regardless of route, those on short hops could pay less than passengers flying cross-country.

And it makes sense for airlines to encourage travelers to pay for their checked bags in advance, he said, in part because that allows the businesses more time to plan staffing levels, cargo allowances and fuel needs.

“But at the same time this introduces a level of added complexity for travelers and uncertainty,” he said. “Trying to figure out what your checked bags will cost becomes more important and could eat into your total travel budget.”

Airlines say that when customers pay for bags in advance, it saves time at the airport and reduces lines. JetBlue, which adjusted its checked bag fees in March to charge more during peak travel times, said the changes were meant to help the airline cover the higher cost of transporting bags and return to profitability.

“By adjusting fees for added services that only certain customers use — especially during periods of highest demand for limited space in the cargo hold — we can keep base fares as low as possible and ensure customer favorites like seat-back TVs and high-speed Wi-Fi remain free for everyone,” the airline said in a statement.

Even before those changes, computer science professor Ravid Shwartz Ziv discovered that paying for his carry-on bag at the airport would come with some sticker shock: a $65 charge because he had booked JetBlue’s cheapest fare. On a later trip, he made sure to pay for his bag earlier at a cheaper price — but he said airport workers still made him pay the $65 amid confusion over his initial booking.

Making matters worse, he said his credit card flagged his attempt to pay at the airport as fraud and he was locked out of using it, ultimately needing a friend to cover the cost so he could catch his flight.

JetBlue recently announced it will no longer charge those who pay the cheapest fare for carry-on bags to “increase customer satisfaction.” Even so, “I probably will not go with them again,” Shwartz Ziv said.

For Nicole Willard, a preschool director in Portola Valley, Calif., bag fees are an essential part of her calculations when flying to visit family in Massachusetts, where she’s from. She often flies back with a checked bag full of favorite gluten-free meals and snacks from home. A JetBlue credit card holder, she was used to checking one bag for free and paying $45 for the second. Then, before a flight last year, she discovered that cost had gone up to $50 — posting on X that it was “just absurd.”

“I was a little peeved about it,” she said.

Citing the tendency of extra fees to “confuse consumers,” the Department of Transportation recently finalized a rule that will force airlines to disclose prices up front — at the first point where a fare and schedule are shown — for bags and for canceling or changing a reservation. Several airlines and an industry group have sued the department over the rule, claiming it is outside the DOT’s authority and arguing that it would confuse travelers.

“For once, I am speechless,” Transportation Secretary Pete Buttigieg wrote on X in response to the lawsuit.

Airlines contacted by The Washington Post did not answer questions about how they plan to implement the rule. They are required to provide data on the fees to third parties like online travel agencies by Oct. 30, and display the fees themselves no later than April 30, 2025. The DOT will work with airlines to ensure their compliance.

Potter, of Thrifty Traveler, said he wouldn’t expect an airline like Spirit to display all the different prices a bag fee could be depending on when it was purchased along with the fare — just the cost of adding that bag online in the moment.

“I would like to think that there’s an easy enough way for Spirit, for example, to do this,” he said.

Tips to ease the bag-fee blow

For travelers trying to navigate the bag fee maze — or even avoid the fees altogether — Potter recommends sticking to a carry-on whenever possible, especially on airlines that allow one for free.

Passengers on budget carriers, which often charge for carry-ons, should use the airlines’ bag fee calculators to learn what a bag will cost them and decide early on how much they’ll pack.

“Adding it to your ticket as you check out is always going to be the cheapest time to do it,” Potter said. On some airlines, paying at the airport to check or carry on a bag can cost more than $75.

“It gets pretty disgusting in some cases to pay for that bag at the airport,” he said. “You’ve got to do the mental math at the time you’re booking your flight.”