If you’re hitting the road and are booking a hotel room, make sure you know the full price you’ll be paying before you arrive. Authorities recently warned several hotels they weren’t being upfront with their online pricing.

The Federal Trade Commission sent warning letters to 22 hotel operators late last year, saying it appeared their online reservation sites may violate the law by providing a deceptively low estimate of what guests could expect to pay because the quoted prices did not include mandatory fees. The hotels were not identified.

The letters cited complaints received by the agency about hotels charging mandatory fees for use of their gym or pool; for Internet access; and even for newspapers.

“These mandatory fees can be as high as $30 per night, a sum that could certainly affect consumer purchasing decisions,” the FTC wrote to the hotels.

The agency told them these fees, often called “resort fees,” should be included in the total price advertised online.

“Consumers are entitled to know in advance the total cost of their hotel stays,” FTC Chairman Jon Leibowitz said in a statement. “So-called ‘drip-pricing’ charges, sometimes portrayed as ‘convenience’ or ‘service’ fees, are anything but convenient, and businesses that hide them are doing a huge disservice to American consumers.”

Ask about fees like this when you book your hotel stay.