Dallas ? Your bags still fly for free on Southwest Airlines, but if you want a better chance at a window or aisle seat it’s going to cost $10 each way.

Southwest announced Wednesday that customers can pay extra to reserve a spot in the boarding line right behind elite fliers and ahead of families and other travelers. Unlike other airlines, Southwest doesn’t offer assigned seats.

The new offering comes after Southwest introduced new fees for minors traveling alone and for bringing a small pet on board. Southwest still doesn’t charge to check the first two bags, but experts and passengers are starting to wonder whether that’s next.

Southwest, like other airlines, is desperately looking for revenue to offset a slump in traffic, especially among business travelers who usually pay higher fares for last-minute or refundable tickets.

The Dallas-based discount airline lost $37 million in the first six months of this year, and analysts expect that 2009 will be its first unprofitable year since the early 1970s.

Many analysts believe Southwest is passing up hundreds of millions of dollars a year by not charging passengers for all checked bags. CEO Gary Kelly has said bag fees drive customers away, and he has ruled them out at least through the end of 2009. No promises beyond that.

Southwest officials say they’re just charging for extra services that customers want.

“The big difference between (the check-in service) and a bag fee is this is strictly optional,” said Kevin Krone, Southwest’s vice president of marketing.

Southwest is considering other amenities with charges, including in-flight Internet service.

The airline believes the early check-in charge can raise about $75 million a year, on par with Business Select tickets, which cost more but come with extras such as early boarding and a free drink.

“We’d be thrilled if it became in the hundreds of millions,” Krone said.

Bob McAdoo, an analyst for Avondale Partners, is more bullish. He believes the boarding charge could raise $250 million a year.

McAdoo estimates that one-fourth of Southwest passengers could pay the check-in fee. Southwest declined to give an estimate.

The new $10 fee is called EarlyBird Check-in, and it was made available Wednesday for trips beginning today and beyond.

Customers can pay the charge up to 25 hours before their plane is scheduled to take off, and they’ll be moved toward the front of the boarding line.