For 26 large U.S. airlines, so-called ancillary fee revenue accounted for 6.9 percent of their total operating revenue in the third quarter of 2009, up from 4.1 percent a year earlier, the most recently available government data shows.
But major carriers risk alienating customers if they follow Spirit Airlines' lead and impose a fee on carryon bags. The small Florida airline in August will begin charging customers up to $45 to place a bag in an overhead bin.
Other fees haven't stopped people from flying, but many of those fees can be avoided. It would be hard for many travelers to avoid a carryon bag fee.
"We believe it is something that's important to our customers and they value, and we will continue making that available to them at no charge," American Airlines spokesman Roger Frizzell said.
New York Sen. Charles Schumer said Sunday that American, Delta Air Lines, United Airlines, US Airways and JetBlue Airways each have committed to him that they would not institute fees for carryon bags. He said he was hopeful other carriers would follow suit.
Notably absent from the list was Continental Airlines, which is said to be in merger talks with United.
It wasn't immediately clear how long the airlines had pledged not to charge for carryons.
Frizzell couldn't say, and a spokesman for Delta declined to comment.
Schumer said he planned to meet with Spirit Airlines leadership in the coming week.
He will have an uphill battle changing Spirit's mind, however.
Ben Baldanza, Spirit's president and CEO, told The Associated Press on Sunday that his airline still plans to go forward with its carryon bag fee.
"Our plan was never predicated on anyone matching us," Baldanza said. "The fact that other people are saying they won't has never changed our view that this is right."
He said the decision by the five major carriers actually puts pressure on those airlines because Spirit has lowered its fares more than the price of the new fee.
"We knew we took a risk with this strategy, but we believe on balance it's one that our customers will buy into," Baldanza said.
Schumer and five other Democratic senators -- Jeanne Shaheen of New Hampshire, Ben Cardin of Maryland, Amy Klobuchar of Minnesota, and Robert Menendez and Frank Lautenberg of New Jersey -- are supporting legislation that would tax airlines if they charged carryon bag fees.
Schumer said the legislation would move forward until it becomes clear that no airline will institute the charges.