Shopping for low airfares is a good way to lower the cost of your trip. But increasingly, the lowest airfare might not work out to be the cheapest trip after all.

That's because cash-strapped airlines are creating new revenue streams with a host of charges for products and services that have been free for many years.

To be clear: we're not talking about the security fees everyone imposed after the September 11th attacks.

Fuel surcharges and airport landing fees are also adding up, but that's a separate issue.

The latest trend is airline-specific charges for a variety of things that were previously free. Sometimes the fees start with one airline and spread to others--prompting travelers to wonder if some of these charges are bound to become the industry standard rather than the exception.

A few disclaimers are in order: it is not an exhaustive list, and we're not picking on any airline listed here. They're operating a business and they've determined the public will pay these fees.

But it's my guess that these fees (and others like them) often catch travelers by surprise. So, in the interest of budget planning, here are a few examples.

Let's start with baggage fees.

In the past, you've had to pay extra for bags that were overweight or oversized. Those fees are increasing, which isn't surprising given the rise in fuel costs.

But now you must also pay a fee on some airlines for the privilege of even checking a bag, no matter its weight or size.

The fee structure for extra bags or large bags has become grotesque in its complexity. Take a look at 20 examples among budget airlines and majors that are watching your packing habits like never before.

Want to select your seat online in advance of your flight? It's a nice service that developed with the Internet, allowing you to reserve a place on the plane from the comfort of your home computer.

Budget carrier AirTran is among the airlines charging for this privilege. The current rate is $6 USD per seat.

That can add up very quickly: if you have a party of three flying a one-stop trip in each direction, you'll pay $72 to reserve all the seats on all the flight segments. Would you like an exit row seat, with the additional leg room it provides? That costs $20. Is it worth that much to you to choose seats?

Allegiant Air also charges for seat assignments ($12/seat). They offer some very nice airfares within the U.S. But even if you choose not to reserve a seat for $12, you'll pay a "convenience fee" of $11.50 for simply making a reservation through AllegiantAir.com or through their call center. It appears the only way to avoid the reservation fee is to book your flight in person at the airport.

Care for a blanket in flight? On Air Canada, it can cost some passengers $2 CAD to feel warm and comfortable. For the same price, Air Canada will sell you a bag of carrots.

The Boston Globe reports JetBlue is charging for pillows and blankets. They also report JetBlue has instituted a $1 charge for headphones.

Starting to get the a-la-carte picture?

US Airways has posted Dividend Miles redemption fees starting at between $30 and $50. The airline also has a plan to charge passengers $2 for soft drinks.

The list could continue for pages, and there are plenty of airlines charging similar fees that are not listed here.

Why is it happening? It's a frequently asked question, and here's one answer from US Airways:

"We estimate that the airline industry will lose over $7 billion in 2008. With fuel costs higher than they’ve ever been, we’ve had to look at ways to offset our increased operating costs. Increasingly, airlines are charging for a-la-carte services such as checked bags, seat assignments, and award travel in order to offset these increased costs."

Fees don't end with the airlines. Two airports in Great Britain charge passengers for using the loading zone in front of their terminals. At Birmingham Airport £1 buys 15 minutes, while London Luton charges £1 for 10 minutes of unloading. Drop-off points further from the Luton terminal are served by a free shuttle, so the fee can be avoided.

Bottom line: it's likely these new fees are here to stay, even if the economic climate for airlines improves. If you lack packing skills or you feel you need these services, budget accordingly. Expect the list of fees to grow.