Spirit Airlines raked in more than $140 million in fees between 2008 and 2011, according to a new study of financial documents by personal finance website NerdWallet.

The news comes less than a week after Spirit was slapped with a class-action lawsuit alleging that its "Passenger Usage Fee," essentially a $9-$17 per flight charge that the airline levies on all tickets not purchased at airport ticket counters, is deceptive.

Lawyers in the case suggested that millions of passengers could potentially be entitled to join the class action, which alleges that the fees are nothing more than a sneaky way for the airline to increase the fares it charges passengers.

And the amount of money Spirit is making from those fees is only going up, writes Alicia Jao, VP of Travel at NerdWallet, citing the airline's public SEC filings as her sources:

In 2006, Spirit’s average ticket revenue per passenger flight segment was $109.36. As of June 30, 2012, average ticket revenue per passenger flight segment has only grown 19% to $130.54. Non-ticket revenue per passenger flight segment, on the other hand, has grown 974% over the same time period. Non-ticket revenue is primarily composed of extra fees, such as baggage fees, PUFs and seat selection fees.

While baggage fees and seat selection fees are indeed optional, lawyers involved in the case against Spirit say that paying the PUF isn't realistically optional because the only way to avoid it is by purchasing tickets at the airport. The suit alleges that Spirit raked in at least $40 million in PUF charges in 2011 alone.

"I am sure the company has additional details around the PUFs," Jao writes in an email to HuffPost Travel, "As well as the more recent Department of Transportation Unintended Consequences fee (initiated in early 2012) that is extremely reminiscent of the PUF but completely unavoidable from our research and inquiries to the company. However, publicly available data is already quite telling."

Here's an infographic look into the nitty gritty so far:

Spirit Airlines' Passenger Usage Fee
Via: NerdWallet