A United Airlines jet approaches Los Angeles International Airport in 2013. Sadly, the moon couldn't afford a seat. (AP Photo/Nick Ut, File)

Online travel sites such as 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 fee of 16 euros (about $18) 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 its seats off third-party sites, via what executives have called "a very pure online distribution strategy," as a way to force interested fliers 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.

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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 with 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 at 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 those of its competitors.

Jean Medina, a spokeswoman 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 Web site 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 e-mails. 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."

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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 gives airlines more control over what they sell (or upsell) to passengers, including bag-check, meal or legroom upgrades, frequent-flier cards and priority seats.

As if all that extra money wasn't persuasive enough, airlines gain another big advantage from keeping passengers on their sites: 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, one 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."

No U.S. airlines have yet to indicate that they'll be following Lufthansa's lead. At a Deutsche Bank conference in Chicago last week, Delta President Edward Bastian said the airline had its booking-fee spending under control after years of slashing costs.

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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.)

But the travel-site industry has defended its growth as a check against high ticket prices and low competition in air travel, especially after years of bankruptcies and megamergers. America's four big airlines — American, Delta, Southwest and United — fly about 80 percent of all domestic passengers, and with the help of cheap jet fuel are posting some of their biggest profits in history.

Those sites — which include both online travel agencies that book tickets themselves, such as Expedia and Priceline, and airfare search engines that help shoppers find where to buy, for example Kayak and Hipmunk — obviously have a high-priced dog in this fight. Their entire business model depends on the rapid flow of public air fares and flight details.

Those companies, too, have been embarking on their own competition-reducing, megamerger spree. Orbitz shareholders last month signed off on a $1.3 billion takeover by big rival Expedia, which also owns Hotels.com and Hotwire.com. Meanwhile, other services are rising to take their place: Even Google now runs a flight search.

There's nothing to stop the astute traveler from bargain hunting across a number of sites. But search and travel sites can often find deals in places many travelers might otherwise miss, such as booking different legs of a trip on competing airlines.

The confusion and hassles have led even to a cottage industry of helper sites like Booking Guru, a "travel concierge service" that passengers can pay to help them book flights or upgrades with frequent-flier miles, starting at about $50.

But the airline trade group said carriers' blocking of flight information wasn't an attempt to remove choices from airplane deal-hunting. If anything, Medina said, it was a choice in itself: A decision airlines had already made for their passengers.

"There are no restrictions on choice," Medina said. "Airlines also need the freedom to choose which third-parties they do business with in order to ensure their product is displayed properly and most cost-effectively. Ultimately, that choice is what will benefit passengers."