The new fees took effect Sunday. Spirit spokeswoman Misty Pinson says initial signs look promising.
Since airlines started charging for checked luggage, more flyers have brought carry-on bags on board. Spirit's new fee is aimed at discouraging passengers from stuffing the overhead bins because it often takes extra time to get bags settled.
Spirit is charging $30 at check-in for any bag that's too big to fit under the seat. The price goes up to $45 for passengers who wait until they get to the boarding gate to pay.
"The check-in process is going well so far," Pinson said Sunday afternoon. "It looks like this is going to speed things up."