Last year, Frontier Airlines announced that only flights booked at FlyFrontier.com will earn 100 percent of miles flown. When booking with a third party, you earn 50 percent of the miles flown. Now, the number of third-party sites selling Frontier Airlines tickets is shrinking. Passengers will no longer be able to book flights on Frontier Airlines at Expedia. You will still find Frontier Airlines flights listed at other online travel booking sites, such as Kayak, Priceline, Orbitz and Travelocity.