MINNEAPOLIS — As if charging $15 to check a bag weren't enough, two airlines are asking for $5 more beginning this summer if you pay at the check-in counter — a fee on top of a fee.

Of course, you could always pay your baggage fee from home. The airlines call it the "online discount."

If airlines can get away with that, what's next? Rather than raise fares in the middle of a recession, they're piling on fees to make money — fees for bags, fees to get through the line faster, even fees for certain seats.

United Airlines alone expects to rake in more than $1 billion this year in fees ranging from baggage to accelerated frequent-flier awards. That's more than 5 percent of its revenue.

The most likely new fees are those that some airline, somewhere, has tried. Fees usually originate with one or two airlines, and competitors watch to see whether passengers accept them or revolt. For instance:

• US Airways and United are hitting passengers up for $5 to pay their baggage fees at the airport instead of online. United implemented the fee June 10, while US Airways will put it into effect July 9.

• If you want to select an exit row seat on AirTran and enjoy the extra legroom, expect to cough up $20.

• Allegiant Air, a smaller national discount airline, charges a $13.50 "convenience fee" for online purchases, even though most other carriers encourage purchases direct from their Web site.

• European discounter Ryanair charges for something everyone has to do if they want to fly: check in. It's 5 euros, or about $6.75, to check in online, double for passengers who pay at the airport. Ryanair plans to eliminate airport check-in desks.

• Spanish airline Vueling charges a fee to pick a seat. A "basic" seat behind the wing runs 3 euros. For 30 euros, travelers can choose an aisle or window seat and guarantee that the middle seat will remain empty.

"They need to chill out with those," said a frustrated Jim Engineer, a public relations executive waiting for a flight out of New York's LaGuardia. "Charging for a glass of water and seats just translates into unhappy customers."

As recently as last year, most fliers only came across a fee if they checked three bags or sent a child across the country. Most people, most of the time, traveled fee-free.

But that began to change last spring. Spiking jet fuel prices and passenger resistances to higher fares started airlines looking around the cabin for things they could charge extra for.

Passengers are finding it's a lot easier for the airlines to add the fees than to take them away.

"They're going to keep nudging them up until they run into market resistance," said Ed Perkins, a contributing editor at the Web site Smarter Travel.

That's what happened at US Airways. It tried for seven months to charge for soda and water but gave up in March after no other airlines took up the idea. And Delta scaled back a plan to charge $50 to check a second bag on all international flights. Instead, the charge will apply only on flights to Europe.

Ideas for fees don't come out of thin air. Last month in Miami, most of the big U.S. carriers and many overseas airlines attended a conference devoted to a-la-carte pricing and fees.

Some fees stretch the imagination: The CEO of European discount carrier Ryanair has floated the idea of charging for lavatory use and sick bags. But even he hasn't gone ahead with what appears to have been a publicity-seeking gambit, and no other carrier has suggested such a charge.