Q. I read recently that you can save money by booking online directly with the airlines rather than on a site like Expedia, even though the contracts between the airlines and the online travel agencies prohibit the airlines from offering lower rates on their own sites. Allegedly the airlines circumvent this restriction by offering discount codes in emails to their frequent fliers or through Facebook and Twitter or through email. I don't mind going directly online to the airline's web site but is there a way for the rest of us who are not frequent fliers to find discount codes?

A. You’re correct that airlines are using promo codes to entice passengers to book directly on their sites, but I'm not sure that airlines are prohibited from selling fares for less on their own sites, since they do this all the time. We see frequent 10% to even 50% off promotions, and Spirit Airlines always sells fares on its own site for less than on third-party sites. Airlines hate paying fees to computerized reservation systems or to travel agents, whether online or “bricks and mortar”. In fact, it’s one of their largest expenses, after fuel, airplane purchases, and labor costs. The beauty of using an online travel agency is that you can easily compare prices, and you’ll learn if it’s cheaper to fly out on one airline and back on another, something that an airline website won’t tell you, so don't give up on them. Price aside, you can also find a range of convenient flight times among several carriers at a glance by using Expedia, Orbitz, Travelocity and their ilk. To learn about promo codes, sign up for your favorite airlines’ emails. The Airfarewatchdog Blog also lists current airline promo codes.