Spirit Airlines currently charges $40 for carry-ons, which came as a surprise to one passenger.
"If you put anything in an overhead locker it costs an extra $40, which is outrageous," said passenger Nattu Mazitelli, a tourist from Melbourne, Australia.
The airline considers carry-ons to be any bag that needs to fit in the overhead bin. Any bag that can fit under the seat is free.
Spirit Airlines is known for add-on fees. The carrier says it allows fliers flexibility: Pay a minimal fare and then add for bags.
Spirit says the $100 fee is reduced to $35 if you pay it online and is slightly higher if you pay at an airport kiosk. The traveler who will be walloped with a $100 charge is the one who books through travel websites.
The sites will show you the cheapest flights but they do not tell you about a carrier's add-on fees. If you don't look them up, you could show up at the gate at Spirit Airlines with a carry-on and a $100 charge.
The advice from fliers is to plan ahead. Mazitelli said he'll stuff extra items under the seat or carry them on his lap if that's what it takes to avoid the carryon fee.
"It is not going to be comfortable but it is only a short flight," he said.
The Associated Press contributed to this report.