Your bags fly free, but only if they go under the plane.
Spirit Airline will start charging passengers for carry-on luggage on flights after July 31. Spirit customers can go online and order a carry-on voucher for $30, or buy one at the gate for $45.
"Airlines are sort of like dominoes. If one of them runs out there and does it, especially a bigger one, the other ones sort of fall in behind," Seaney said.
A spokesperson for American Airlines said the carrier can't discuss what "we may, or may not, do in the future -- only what we have actually done."
"Let me say that our competitors continue to distinguish themselves apart from Southwest Airlines and make it very easy for this company to maintain the integrity of the bags fly free message," Hawkins said.
"People expect a certain level of accommodation that comes with what you get. What you buy is what you get and this kind of strategy just doesn't resonate with customers and we've seen seen that success of our own campaign. There are certain things that just come with the experience and Southwest Airlines has really stuck with this bags fly free messaging and it's been very successful and it's really starting to set us apart," Hawkins said.
Seaney said if other carriers start charging for carry-on bags, some passengers will try other ways to save money.
"I think we'll see trench coats with socks and underwear with a variety of other things. People will find all types of creative ways to get around these fees," Seaney said.