Baggage fees continue to grow in importance as a revenue source for US airlines, which collected a record $4.9 billion in checked-baggage charges last year, the Atlanta Constitution-Journal reports.

That’s up from the $2.6 billion airlines collected from baggage fees in 2017, according to US Bureau of Transportation Statistics data.

American Airlines alone generated $1.2 billion in baggage-fee income in 2018, followed by United Airlines ($888.7 million) and Delta Air Lines ($788.5 million). Spirit Airlines and Frontier Airlines had the next-highest revenues deriving from checked baggage.

Interestingly, Delta’s fee revenue was down sharply from 2017, when the airline collected $907.6 million in bag fees. That may be due to more travellers being exempt from fees, such as the one million Delta American Express gold customers that the airline signed up each year in 2017 and 2018.

Airlines benefitted from an industry-wide trend of charging more for checked bags. In 2018, for example, United, Delta, Jetblue and other carriers raised the price of a first checked bag from $35 to $40.

Southwest Airlines is a notable exception in that the airline does not charge any fees for the first two checked bags. Nonetheless, the airline generated $49.9 million in checked-bag fees last year.