A new class-action lawsuit alleges that low-cost carrier Spirit Airlines has duped consumers out of millions of dollars, presenting a so-called Passenger Usage Fee as a government-required charge rather than a profit-driving ancillary fee.

Lawyers from the firm Podhurst Orseck say in a release that the airline "has intentionally and systematically targeted consumers with deceptive advertising and pricing practices for years."

"It's an illusory fee," firm partner Kathy Ezell told The Huffington Post. "It's really a tack-on to the fare, it's to increase their profits and, under the DOT regulations, it should be disclosed so that the customers know what they're paying for the fare. Instead they have chosen to embed it with other fees that are either required or sanctioned by the government -- and they give it an innocuous name -- so that it sounds like one of the other fees."

Spirit currently lists the fee, which varies from $9 to $17 per flight, as "optional" on its website, under the headline "Booking related fees." (Ezell says that hasn't been the case in the past.)

But Spirit does not currently appear to give the option to avoid the fee, which is broken out in itineraries as "Passenger Usage Fee" and listed as an unavoidable part of the base fare. (Here's one example from a hypothetical September itinerary between New York and Detroit.)

"Their excuse for charging the fee is to provide a website and phone service for passengers to purchase tickets and that's ludicrous," Ezell says. "You're not getting any good or service in exchange for the payment of that fee."

According to the firm, as many as 5 million passengers might be eligible to join the class action, based on DOT passenger statistics and lawyers' estimates of passengers who would've paid the fee.

"Spirit believes the claims are without merit and intends to defend the case," spokeswoman Misty Pinson told The Huffington Post in an email.

Ezell counters that Spirit has historically and habitually been cited by the DOT over the Passenger Usage Fee and fined -- to the tune of $375,000 in September 2009, to give just one example.

"They put it up and take it off periodically," Ezell says of the fee. "We don't know, but we think it's possible that they've made as much as $40 million in profit, so, yeah, they're happy to pay a $375,000 fine and just continue the practice."

The suit comes at a time when government regulators are moving to clarify ticket prices -- and airlines rake in billions of dollars in ancillary fees for services like checked baggage and seat selection.

In March, Spirit CEO Ben Baldanza defended his company's practices in a blog post on The Huffington Post entitled "Federal Transparency Rules Are Bad For Travelers."

In the post, Baldanza writes, "The DOT has good intentions for the public, but on these issues they simply misfired. Many who oppose our view have no real concern to keep fares low for ordinary consumers, but are looking out for large corporate interests." He also writes that "[Spirit's] optional pricing structure has saved customers millions by allowing them to pay only for what they want. Many fly Spirit who could not afford to fly on other airlines -- and our 'optional pricing' model creates this opportunity."

It's not the first legal challenge over fees for the carrier this year, which was hit in March with a suit alleging that its $2 "unintended consequences" fee -- so named because it was blamed on new DOT regulations -- was illegal under Illinois consumer protection laws. The plaintiff in that case said the fee was "nothing more than a profit-generating device."

The latest case was filed in United States District Court for the Southern District of Florida. Spirit is headquartered in Miramar, Florida, near Miami.

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  • Blanket Fee

    Back in 2010, American Airlines <a href="http://www.huffingtonpost.com/2010/02/08/american-airlines-to-char_n_454109.html" target="_hplink">started charging $8 for blankets</a>.

  • Ryanair Boarding Pass Fees

    Songstress Lily Allen took to twitter to express anger over<a href="http://news.travel.aol.com/2011/06/06/lily-allen-twitter-ryanair-boarding-pass/" target="_hplink"> Ryanair's policy of charging passengers to print out their boarding passes</a>.

  • Spirit's Online Booking Fee

    In November 2011, Spirit Airlines <a href="http://www.huffingtonpost.com/2011/11/14/spirit-airlines-raises-on_0_n_1093430.html" target="_hplink">raised its domestic "passenger usage fee" (aka online booking fee) from $8.99 to $16.99</a> each way.

  • Fuel Fees

    Southwest Airlines <a href="http://www.huffingtonpost.com/2012/03/26/southwest-airlines-fees-fuel-costs_n_1381008.html" target="_hplink">raised its ticket prices by $4 to $10 to offset the high cost of jet fuel</a> in March 2012. Its subsidiary AirTran, plus United Airlines, Delta Air Lines, American Airlines, US Airways, Frontier Airlines and Virgin America followed suit.

  • Ryanair Emergency Row Fee

    Ryanair found itself under investigation after <a href="http://www.huffingtonpost.com/2012/03/20/ryanair-in-hot-water-over_n_1366497.html" target="_hplink">instituting a 10 pound fee to sit in the emergency row</a>.

  • Allegiant Air's Carry-On Fee

    In April 2012 the budget carrier announced a <a href="http://www.huffingtonpost.com/2012/04/02/allegiant-air-carry-on-fe_n_1397911.html" target="_hplink">$35 carry-on fee</a>.

  • Spirit's New Carry-On Fee

    A month later, low-cost Spirit Airlines <a href="http://www.huffingtonpost.com/2012/05/02/spirit-airlines-carry-on-fee_n_1472508.html" target="_hplink">upped carry-on fees to as much as $100</a>.

  • Airlines Could Charge Extra For Seats Together

    Late May 2012 saw airlines start to <a href="http://www.huffingtonpost.com/2012/05/21/airline-charges-seats_n_1533866.html" target="_hplink">reserve more window and aisle seats for passengers willing to pay extra</a>. This would make it it harder for friends and family members to sit next to each other.<br /> <br /> Sen. Chuck Schumer urged airlines to <a href="http://www.huffingtonpost.com/2012/05/27/chuck-schumer-airlines_n_1548794.html" target="_hplink">allow families with young children to sit together without paying extra</a>.

  • United's $100 Bag Fee

    In June 2012, United <a href="http://www.huffingtonpost.com/2012/06/12/united-airlines-raises-international-bag-fee_n_1589223.html" target="_hplink">raised its fee for a second checked bag on trans-Atlantic flights to $100</a>. Delta had done the same a few months earlier.

  • Wizz Air's Carry-On Fee

    Carry-on fees have finally hopped the pond. <a href="http://skift.com/2012/07/09/carry-on-bag-fee-spreading-wizz-air-charge-europe/" target="_hplink">European regional carrier Wizz Air instituted a 10 Euro (about $12) fee to use the overhead bins</a>. Bags that fit under the seats still fly free.

  • Credit Card Booking Fee

    In August 2012, Airefarewatchdog called out <a href="http://www.huffingtonpost.com/george-hobica/the-latest-airline-fee-credit-card_b_1829396.html">Allegiant Airlines for charging more to book flights via credit card</a>.

  • Southwest Airlines' Early Boarding Fee

    <a href="http://www.huffingtonpost.com/2013/01/22/southwest-airlines-fees_n_2525443.html?utm_hp_ref=travel#slide=more232494">Southwest passengers can pay $40 to be one of the first 15 people on the plane</a>, as of January 2013.

  • United Fare Increase

    United Airlines announced in December 2012 that it would be raising domestic fares up to <a href="http://www.huffingtonpost.com/2012/10/10/united-airlines-raises-prices-10-dollars_n_1954864.html" target="_blank">$10 per round trip</a>. While the price bump is minimal, travelers looking for the best deal could be dissuaded from purchasing the slightly more expensive tickets.

  • Allegiant Air

    One of the most profitable airlines in the U.S., <a href="http://www.huffingtonpost.com/2013/06/28/allegiant-air-fees_n_3516384.html" target="_blank">Allegiant Air</a> came into the spotlight for its length list of added-on fees. The budget airline is similar to Ryanair in wooing travelers with low-cost flights to small airports and tacking on hidden fees in every aspect of the flight. In addition to the run-of-the-mill luggage and seat-choice fees, Allegiant has fees for paying with a credit card ($8), using the overhead luggage compartments ($10-$25) and booking over the phone ($50!). (AP Photo/David Becker)

  • American Airline Fare Initiatives

    In response to "nickle-and-diming" complaints, American Airlines introduced new <a href="http://www.huffingtonpost.com/george-hobica/american-airlines-new-fare_b_2312289.html" target="_blank">fare-bundling initiatives</a>. The options -- "choice essential" and "choice plus" -- offer an array of packaged perks for one set price, rather than a la carte. While many complained about the initiative, others saw it as a way to get more for your money.

  • United Premier Access

    United Airlines rolled out "<a href="http://www.huffingtonpost.com/2013/03/01/united-premier-access-rip-off_n_2788513.html" target="_blank">Premier Access</a>" in March 2013. The program includes a designated check-in and security lines, and priority boarding and bagging handling. While the fees for the exclusive services only start at $9, travelers must <em>already</em> be elite customers.

  • Frontier Third-Party "Fees"

    In May 2013, <a href="http://www.cntraveler.com/daily-traveler/2013/05/frontier-airlines-carry-on-fees-050213?MBID=twitter_" target="_blank">Frontier announced</a> that anyone purchasing a ticket through a third-party, such as travel agents or websites like Expedia, would be subject to additional fees. In reality, the fees are actually perks (such as carry-on luggage) Frontier only offers to travelers booking through the airline directly.

  • New Alaska Airline Fees

    In July 2013, <a href="http://www.huffingtonpost.com/2013/07/09/alaska-airlines-fees_n_3568206.html" target="_blank">Alaska Airlines</a> raised its baggage checked fee from $20 per bag (for up to three bags) to $25 per bag for the first two bags, and $75 for an additional piece of luggage. The airline also upped it's ticket-change fee to $125.

  • Spirit's Potential New Fees

    Spirit Airlines is mulling the idea of tying airfare fees to demand. In October, <a href="http://www.huffingtonpost.com/2013/10/31/spirit-airlines-fees_n_4181354.html" target="_blank">the AP reported</a>: "Spirit Airlines is considering tying the fees passengers pay to check a suitcase or pick a more desirable seat to demand. On a peak travel day, for instance, the fees could be much higher. Passengers who booked a Spirit flight for this holiday season can relax however — the changes are months away, if they happen at all."

  • Virgin Atlantic Seat Reservation Fee

    <a href="http://www.huffingtonpost.com/2013/12/17/virgin-atlantic-seat-reservations-fee_n_4459610.html" target="_blank">Virgin Atlantic</a> will start charging travelers $40.65 (£25) per flight to reserve their seats more than 24 hours in advance. Those who aren't picky about where they sit can choose their seat less than 24 hours before the flight at no cost.