According to The CarTrawler Yearbook of Ancillary Revenue by IdeaWorksCompany, last year airlines collected $40.5 billion in ancillary revenue—ticket change fees, penalties, and upgraded seat fees.

Which airline earned the most in added fees? United, at $6.2 billion.

Who made the most per passenger in ancillary revenue?

Not surprisingly, Spirit Airlines raked in nearly $52 per passenger, per flight.

These fees add 43 percent of additional revenue to some airline’s bottom lines.

But you know what that means—those fees will remain, but revenue from those fees (if airlines have their way) will continue to grow

Keep reading to see how much each airline earned in 2015:

Alaska Air Group

Ancillary Revenue: $1,092,000,000
As a % of Revenue: 19.5%
Dollars per Passenger: $34.25
Total Revenue: $5,598,000,000
Passengers: 31,883,000

Allegiant Air

Ancillary Revenue: $474,494,000
As a % of Revenue: 37.6%
Dollars per Passenger: $49.94
Total Revenue: $1,262,188,000
Passengers: 9,500,611

American Airlines

Ancillary Revenue: $4,718,000,000
As a % of Revenue: 11.5%
Dollars per Passenger: $23.44
Total Revenue: $40,990,000,000
Passengers: 201,249,000

Delta Air Lines

Ancillary Revenue: $3,775,102,000
As a % of Revenue: 9.3%
Dollars per Passenger: $21.04
Total Revenue: $40,704,000,000
Passengers: 179,382,874

Frontier Airlines

Ancillary Revenue: $288,000,000
As a % of Revenue: 18.0%
Dollars per Passenger: $23.55
Total Revenue: $1,604,000,000
Passengers: 12,231,000

Hawaiian Airlines

Ancillary Revenue: $213,470,150
As a % of Revenue: 9.2%
Dollars per Passenger: $20.02
Total Revenue: $2,317,467,000
Passengers: 10,665,000

JetBlue

Ancillary Revenue: $812,700,000
As a % of Revenue: 12.7%
Dollars per Passenger: $23.15
Total Revenue: $6,416,000,000
Passengers: 35,101,000

Southwest Airlines

Ancillary Revenue: $2,118,600,000
As a % of Revenue: 10.7%
Dollars per Passenger: $14.65
Total Revenue: $19,820,000,000
Passengers: 144,574,882

Spirit Airlines

Ancillary Revenue: $928,369,000
As a % of Revenue: 43.4%
Dollars per Passenger: $51.80
Total Revenue: $2,141,463,000
Passengers: 17,921,000

Sun Country Airlines

Ancillary Revenue: $44,154,000
As a % of Revenue: 8.4%
Dollars per Passenger: $23.85
Total Revenue: $526,000,000
Passengers: 1,851,000

United Airlines

Ancillary Revenue: $6,199,000,000
As a % of Revenue: 16.4%
Dollars per Passenger: $44.16
Total Revenue: $37,864,000,000
Passengers: 140,369,000

Virgin America

Ancillary Revenue: $166,724,656
As a % of Revenue: 10.9%
Dollars per Passenger: $23.70
Total Revenue: $1,529,584,000
Passengers: 7,036,000

For more information about airlines and airports, check out:

• How Much Airlines Charge for Checked & Carry-On Luggage
• What A New Law Will Change About Air Travel
• Is Base Airfare a Better Deal?

Keep reading for more travel tips.