For the extra $10 fee each way, Southwest said Wednesday that passengers can reserve their boarding spot while buying their ticket. Under the Dallas-based low-fare carrier’s current policy, passengers can begin to check in 24 hours prior to their flight. Passengers who check in the earliest get to board first.

Boarding on Southwest is unlike most airlines where passengers can reserve a seat on the plane.

Southwest (NYSE: LUV) spokeswoman Whitney Eichinger said the new option allows passengers to not have to worry about checking themselves in.

While many airlines have been charging passengers fees for beverages and baggage, Southwest has aggressively marketed its “no frills” consumer policies to set itself apart. And what sets this option apart, Eichinger said, is that, in contrast to Southwest’s competitors, passengers have a choice of paying the $10 fee.

“These are opportunities that customers can have the option of taking advantage of, instead of being forced to pay the additional fee,” she added.

Southwest, the No. 2 carrier at Lambert-St. Louis International Airport, said Tuesday that it is adding service from St. Louis to Boston and Minneapolis but also trimming flights to Baltimore, Detroit, Cleveland and Las Vegas.