A Florida law firm has filed a class action suit against low-cost carrier Spirit Airlines over its so-called "passenger usage fee." While consumer resentment over Spirit's excessive reliance on various fees is hardly new, what's interesting about this fee is that the Transportation Department has judged it to be an unfair and deceptive trade practice — twice — and the airline had agreed both times to stop breaking it out from its base fare.

On its website, Spirit lists the Passenger Usage Fee among its "optional fees." it ranges from $8.99 to $16.99 per customer, each way. What's it for? Passengers must pay the fee if they book their flight over the phone or online; the only way to avoid it is to book and buy a ticket at the airport counter.

The class action filed in Florida calls Spirit's Passenger Usage Fee "fraudulent, deceptive and unconscionable."

Apparently the U.S. Department of Transportation agrees. In 2008, DOT fined Spirit $40,000 for charging a passenger usage fee "which the carrier assessed on all tickets not purchased at an airport ticket counter," the agency said in its enforcement order. As part of that settlement, Spirit agreed to "cease and desist" from assessing such a fee.

In 2009, DOT took another enforcement action against Spirit — with an even larger fine — for a number of various rule violations, including that passenger usage fee once again. And the airline again agreed to clean up its act.

Yet the charge is still there, listed as an optional fee, even though it's unlikely many travelers will go all the way to the airport to buy a ticket just to avoid paying it. The law firm that filed the class action against Spirit said the airline may have collected as much as $40 million from the questionable fee.

Earlier this year, another lawsuit against Spirit filed in Illinois challenged a new $2 per segment fee, which the airline calls the "Unintended Consequences of DOT Regulations" fee. The airline said that fee covers the costs it incurs as a result of DOT's new rule that guarantees travelers a 24-hour window after booking to change their plans without a penalty.

A new analysis by the website nerdwallet.com notes that Spirit has been relying increasingly on passenger fees to generate revenue, with considerable success. It said that the airline's non-ticket revenue per passenger rose from $4.80 per flight segment in 2006 to $51.57 in 2012.

And those fees are going up: Effective November 6, Spirit will raise a number of its fees, including its notorious fee for carrying on a bag and storing it in the overhead bin. The carry-on fee will rise from $30 to $35 if it is paid online at the time of booking. If the passenger waits until he's at the gate or kiosk to pay for a carry-on, the current $40 fee for a carry-on bag goes up to $50.