Airlines, in a seemingly never-ending bid to supplement their revenues, are adding yet again to an already dizzying array of fees. This summer, both U.S. Airways and United Airlines will charge passengers a $5 service fee if they choose to check their bags and pay the checked baggage fee at the airport instead of online. This is in addition to the $15 both charge for the first bag a passenger checks, and the $25 charged for a second bag.

This fee-on-top-of-a-fee has been in effect since June 10 at United and will go into effect July 9 at U.S. Airways.

Other airlines are adding baggage fees as well. Starting July 1, Alaska Air will charge $15 for the first bag checked. And Delta and Northwest are adding a $50 charge for a second bag checked to Europe, starting on July 1.

Fees have become important revenue streams for airlines that have been struggling as a result of the economic recession. For example, United expects to take in $1 billion in fees during 2009.

Baggage is just one area in which airlines are adding fees. A USA Today survey of 14 major airlines revealed 28 different types of fees, ranging from charges for seats with more legroom to fees for drinks and snacks.

Several business travel organizations have expressed concern that the range of fees is misleading consumers about the actual costs of flying. In a survey of 320 corporate travel managers conducted last fall, the National Business Travel Association reported that 76 percent of respondents believe that “airlines are in fact misleading the public with their ‘low’ and ‘advertised’ fares by adding substantial fees.”

While many travelers resort to carry-on bags to escape the added costs, U.S. Rep. Dan Lipinski (D., Ill.) introduced a bill on June 15 that would, if passed, standardize the size of carry-on bags.

The proposed legislation would ban all carry-ons larger than 22 inches by 18 inches by 10 inches, and calls for the Transportation Security Administration to set up templates limiting carry-ons to those dimensions.

Currently, each airline sets its own standards for carry-on bags. At 50 linear inches, the dimensions in the proposed legislation appear to be smaller than the guidelines set by most airlines.

In other airline news, American Airlines, which offers only food-for-purchase in its main cabin, has stopped taking cash to buy food onboard, and now only accepts debit or credit cards. It joins United, which stopped accepting cash in late April, and Southwest, which has accepted only credit and debit cards since September 2008.