The Transportation Department said the digital travel agent failed to list fuel surcharges and other fees when advertising airfares.
There are so many fees and surcharges in the airline industry that the Department of Transportation has rules to ensure conformity so advertising isn't misleading to customers.
"Many consumers choose their flights based on price, which is why we require all airfare ads to include the full price consumers will have to pay," U.S. Transportation Secretary Ray LaHood said in a statement.
"We will continue to make sure that airlines and travel agents comply with our price advertising rules and will take enforcement action when they do not."
The Transportation Department said its enforcement division found that the Web site's "flexible dates tool" was inconsistent when it listed prices, as some included surcharges and others did not. Further, if flights were listed as cheapest to most expensive, the fares without the surcharges included were put at the top of the list, drawing consumers to falsely advertised prices, the Department of Transportation said.