And that's not to mention that it's becoming common practice to put a premium on aisle and window seats, forcing families that want to sit together to both pay for an upgrade and pay a fee to be assigned seats.

With this new line in the sand comes the suggestion of regulation. U.S. Senator Charles Schumer, a New York Democrat, this week asked for changes in federal policy to regulate airline seating so families aren't forced to have to pay extra to stay together on flights.

"The onerous nature of the fee may well result in children being separated from their parents - and out of sight - to avoid these substantial costs," Schumer wrote to U.S. Transportation Secretary Ray Lahood. "This is unacceptable and it raises safety and liability concerns for the airline, as children potentially have to sit with strangers out of the sight line of their parents in another part of the aircraft."

Jamie Bartosch of Arlington Heights, Illinois, says she and her family were on a recent flight from Grand Cayman Island, where they were in the cramped back row for five hours because they didn't pay the $39 per seat upgrade fee. Meanwhile, other seats sat empty.

All these fees, says Bartosch, who writes a travel blog on TravelingMom.com, have had an impact on families. "To me, all of these fees unfairly hurt family travelers, because it's nearly impossible to travel with kids and put everything into one carry-on. So we are basically forced to pay the luggage fees. To travel comfortably, with food and legroom, now adds at least another $100...Traveling by air is so unpleasant these days, unless you're a first-class business traveler."

The process of charging for this or that is part of an ongoing experiment that has airlines tilting toward frequent flyers and business travelers, who are willing to pay premiums, says airline industry analyst Robert Mann, of Port Washington, New York-based R.W. Mann & Co. And, Mann says, the frequent flyer doesn't want a middle seat.

So-called unbundling - turning a once all-inclusive price into more of an ala carte experience - has led to all sorts of creative interpretations.

"Unbundling can leave customers feeling nickel-and-dimed, but it's a smart idea in that those looking for perks can have them, absorbing a higher share of the cost," says commercial pilot and columnist Patrick Smith, author of Ask The Pilot. "Is it not better to charge a premium for specific items, not all of which everybody wants, rather than raise prices across the board?"

Spirit (SAVE.O), best known for its stripped-down fares, has become the poster child for fees. Here are some they assess: Reservation booking fee, Unintended Consequences of DOT Regulations fee, a passenger usage fee, a fee for agent printing boarding pass, and a fee for print boarding passes at a kiosk.

The planned $100 carry-on fee will apply to those who bring carry-ons to the gate with the hope they'll get checked for free, something other airlines typically do because too many people are trying to avoid checked baggage charges with carry-ons.

"They're playing a game you're going to check the bag and you're going to do it for free," Mann says. "Spirit sees people are playing a game. When you introduce a rule like you have to pay to check a bag or you have to pay to carry on a bag, people change behavior."

Fees are clearly on travelers' minds. The website Travelocity surveyed users about airline fees they expect to pay. About 52 percent said they'd likely pay for a checked bag, while 21 percent said they'd be likely to pay for advance seating assignments. The website Nerdwallet.com recently launched an airline fee search tool that lets travelers compare airline fees in such areas as cost to check bags and seat assignment fees.

The next trend Mann says U.S. passenger will likely see is a move toward the model Air Canada (ACa.TO) has long had in place: Letting consumers choose how much they want to pay for in pre-packaged bundles. That could be anything from a stripped-down set that includes little more than airfare and only one-quarter of the frequent flyer credit, to one that includes premium assigned seats, no added baggage charge, a meal and 50 percent extra frequent flyer credit.

The result, particularly with the seat assignment and premium seating charges (and the abandonment of early boarding for parents with small children), is a divide between the carriers - making airlines such as Jet Blue and Southwest look even more appealing to families since they don't assess such charges, Mann says.

Beyond that, having a policy that can force families to scatter about a plane is potentially dangerous, he says. "I think it's a safety issue." If a child sitting away from his or her parent needs help, "you'd hope someone is assisting that child," Mann says. "They're clearly not responsible for them."

(The author is a Reuters contributor. The opinions expressed are his own)

(Follow us @ReutersMoney or here. Editing by Beth Pinsker Gladstone and Andrew Hay)

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