WASHINGTON – For many passengers, air travel is only about finding the cheapest fare.

But as airlines offer a proliferating list of add-on services, from early boarding to premium seating and baggage fees, the ability to comparison-shop for the lowest total fare is eroding.

Global distribution systems that supply flight and fare data to travel agents and online ticketing services like Orbitz and Expedia, accounting for half of all U.S. airline tickets, complain that airlines won't provide fee information in a way that lets them make it handy for consumers trying to find the best deal.