SANDY SPRINGS, GA—Moments after saving hundreds of dollars on round-trip airfare from Atlanta to Los Angeles, Phillip Walden, 41, experienced mixed feelings about the bargain $39 Southwest Airlines flight. "What sort of corners would they have to cut to make a profit on that low a fare?" Walden wondered aloud after completing the Expedia.com purchase. "Would $39 from every passenger even cover the fuel?" For safety's sake, Walden resolved to buy the second-cheapest ticket available from now on.