Like many resort hotels, the Marriott San Juan Resort and Stellaris Casino in San Juan, Puerto Rico, adds a fee to its daily room rate to cover amenities such as bottled water, a casino coupon, local phone calls and wireless Internet.

And as is the case at many resort hotels, it doesn’t matter whether you drink bottled water, want to visit the casino, make a phone call or use the Internet. Marriott’s fee is mandatory.

Resort fees are routinely hidden on travel and hotel sites, but nowhere, as Steve McEvoy recently discovered, are they more dramatically concealed than on such so-called “opaque” sites as Hotwire and Priceline.

When McEvoy booked a room at the Marriott through Priceline, a site that doesn’t reveal the name of the hotel until you’ve paid for a nonrefundable reservation, he was told that he’d pay only $150 a night. But his e-mail confirmation said that he’d be billed an extra $22 in fees — that, in effect, the surcharge was part of the room rate. “Is anyone trying to write a law to prevent this from happening?” asked McEvoy, a transportation consultant who lives in Philadelphia.

As a matter of fact, yes. The lack of disclosure of these extra charges, a longtime source of frustration for travelers, is getting some attention from a group of consumer advocates led by Ed Perkins, a fellow syndicated travel columnist for Tribune Media Services and a former Consumer Reports editor. In a letter he sent to the Federal Trade Commission last month, Perkins asked the agency to rule that these fees are “unfair and deceptive.” An FTC decision on the matter would close a loophole that collectively costs travelers tens of millions of dollars every year.

The way some resort fees are broken out and disclosed is commonly referred to as “drip” pricing: This means that a company initially advertises only part of a product’s cost, then reveals additional mandatory charges later, as a consumer goes through the buying process. And hotels aren’t the only ones to use this price-tag sleight of hand; you can also find it in the automobile sales and financial services industries, among others.

Drip pricing is a special concern to the FTC. This spring, the agency hosted a workshop on the issue and solicited complaints from consumers, a potential sign that it may soon act to curb this practice. Perkins hopes that the government will start with hotels. A representative for the American Hotel and Lodging Association, the trade organization for the U.S. hotel industry, said that the organization couldn’t speak about the issue until it consulted with its members. The FTC didn’t respond to a request for a comment on Perkins’ letter. A Priceline representative wouldn’t comment on its resort-fee disclosure practices, although in past cases, the company has said that it believes the way it displays mandatory fees after a purchase is sufficient.

Asked about Priceline’s disclosure, a Marriott representative pointed to his company’s website, which prominently shows a resort fee but calculates it as part of the price after a room is selected. Marriott can’t control how these fees are displayed on Priceline, he added. “We provide the rate and applicable fees,” he said. “The online travel agency determines how to display it.”