With all the airlines fees attached to flying these days, Southwest Airlines is still the only U.S. airlines that lets travelers’ bags fly free. And it’s both pieces of luggage that don’t cost an additional dime.

With the majority of carriers tacking on an average of $20 to check one piece of luggage, fees can increase travel costs significantly for a family traveling for the holidays. Airlines have also devised a schedule of fees for what was the normal two bag limit: travelers can pay fees as high as $25 for a first bag and $35 for the second…times two if you’re traveling round trip.

As the travel industry moves into the holiday travel season, the no-fee policy has helped Southwest increase its share of the domestic market by about 1 percent, or $800 million to $900 million.

Southwest, already a low-fare branded air carrier has experienced growth by the successful “bags fly free” campaign. [The campaign] has driven traffic growth at Southwest in a time when we are actually reducing our capacity and the domestic market is actually shrinking,” said Gary Kelly, Southwest’s CEO.

Southwest’s no-fee policy is a gamble at a time when travel overall is down and most carries are cutting back in every area as well as adding fees to compensate for decreased travel. Fees for what used to be free air travel amenities can add up to billions in revenue; checked luggage fees added over $1 billion in the first half of the year for airlines.

Based on Southwest’s growth in the market, its “bags fly free” marketing is a unique selling point that divides them from the rest of the carriers as passengers seek ways to save on travel costs, including considering additional airlines fees.