WASHINGTON – Online travel sites like Expedia and Kayak have made booking and shopping for flights cheaper and easier than ever. And airlines keep finding exciting new ways to slow them down.

Europe’s largest airline giant, the Lufthansa Group, says all tickets starting in September will be charged a 16-euro (about $18) fee if they’re booked on third-party sites. America’s biggest airlines haven’t gone that far, though they’ve crafted their own ways to hamper passengers wanting to scrounge for deals.

Delta has removed its fares from TripAdvisor and dozens of smaller sites, and American Airlines last year briefly scrubbed its flights from Orbitz while fighting over fees. Southwest has for years kept their seats off third-party sites, via what executives have called “a very pure online distribution strategy,” as a way to force interested flyers to keep coming back.

Airlines say their sites carry the most accurate, lowest priced tickets, which they can sell without having to pay booking fees to other online travel agencies. But blocking fares and flight schedules from third-party sites mostly forces hopeful fliers into a complex airline juggling act. All too often, critics say, they make it harder for passengers hoping to compare prices and save cash.

A study last month commissioned by the Travel Technology Association, the industry group for travel sites, said restricting what seat price and schedule information was shared to third-party sites could cost passengers more than $6 billion a year. Not only that, it could also discourage more than 40 million passengers from flying because prices looked too high.

The study, by a Yale professor and former deputy assistant attorney general for the U.S. Department of Justice, said Delta has taken an increasingly isolationist stance by blocking out smaller travel sites. Southwest, the study added, was helping boost its low-fare reputation, even when its tickets were more expensive, by not allowing its fares to be displayed side-by-side with its competitors.

Jean Medina, a spokesperson for industry trade group Airlines for America, defended the third-party wall, saying, “Airlines want to sell tickets, and they want to do so in the least costly way possible, which is why buying tickets directly from an airline website provides the lowest fare.”

She added that the study was based on “fuzzy math,” saying, “Airline pricing is driven by demand and other complex factors. To conclude that carriers’ listing strategies will impact pricing requires a leap of logic.”

Delta did not respond to calls or emails. But in a statement to CNN last month, Delta spokesman Anthony Black said, “Delta reserves the right to determine who it does business with and where and when its content is displayed.”

Airlines have for years tried to shunt business to their own sites, because tickets sold via online travel agencies cost them booking fees, travel-agency commissions and other so-called “distribution costs.”

Those fees are a small part of airlines’ big budgets, especially compared to fuel and salaries. But selling seats through their in-house sites also allow airlines more control over what they sell (or upsell) to passengers, including bag-check, meal or legroom upgrades, frequent-flyer cards and priority seats.

As if all that extra money weren’t persuasive enough, airlines gain another big advantage from keeping passengers on their site: Less competition. On travel sites, the airlines are just one more dollar amount, and not necessarily the cheapest, among a sprawling list of rival airlines. On their own sites, their price is the best, and the only, you can get.

In the case of that $18 surcharge, Lufthansa said it would help the German airline group charge those distribution costs to the passengers who caused them, rather than spreading the fees to everyone.

“It is not about … trying to discourage anyone from comparing prices,” Martin Riecken, a Lufthansa spokesman, told the BBC. “We rather aim for more transparency.”

More than 30 percent of tickets are still booked through airline Web sites, compared with about 20 percent on online travel sites, travel consultancy Atmosphere Research Group says. (Most of the rest comes through business bookings through travel agencies.)