Today, Travelocity dropped its booking fee for airline tickets. A week ago, Expedia did the same.

Both agencies are promising that the fees will be on hiatus until May 31, 2009. But bringing the charge back may be tough: Back in 2007, Priceline and Hotwire dropped their booking fees “temporarily,” and they still haven’t brought the fees back.

That leaves Orbitz as the lone holdout among the biggest U.S. travel agencies. So when will Orbitz, the biggest holdout, throw in the towel on fees?

Consumers should welcome the rollback of these add-on booking charges. But this episode shows how brutal the online travel marketplace is right now. If online travel agencies want to collect a surcharge, they’re going to have to get creative, and earn it. Simply offering price comparisons and a few online alerts — which are free elsewhere — won’t cut it. And Travelocity, Expedia, Priceline, and Hotwire have admitted that.

Sure, Orbitz may counterargue that they provide value-added with their price guarantee, but since that service is of relatively limited value, I wouldn’t pay a premium for it (though maybe it’s worth the $6.99+ gamble for someone else…)

Travelocity is even poking a stick in Orbitz’ eye by copying their “Price Assurance” model and bringing it to vacation packages under the name “PriceGuardian.” If someone else books the same package as you, and the price has dropped, you get a check for the difference. Yeah, good luck with that.

What we may see is a shift to voluntary fees for add-on services, much like the airlines are going a-la-carte themselves. Want a price-drop guarantee? Pay a few bucks up front. Want text message alerts? A few more bucks. That I could see happening. But the standard one-size-fits-all fee is history at the mainstream agencies.