Now you see those summer travel deals. Now you don’t.

Spike Spencer knows what that’s like. He just tried to book a four-night tour online from Icelandair, advertised at $1,073, including flights. But as soon as he selected his vacation, the price jumped to $2,600.

He complained to Icelandair, and it claimed the price was “neither a discrepancy nor a problem.” The company simply ran out of the $1,073 vacation packages.

“When you’re advertising something for a price and there’s a limited number of trips, it’s a bait-and-switch if you don’t have them,” says Spencer, a writer based in Los Angeles. “At least that’s my opinion.”

Michael Raucheisen, an Icelandair spokesman, confirmed that some of the airline’s vacations were in short supply. “Our packages are very popular and they do sell out pretty quickly,” he noted.

Most customers know that when it comes to online sales, timing is everything. But crack open the hood on the travel industry’s deal-marketing machine, and you’ll uncover even more.

I know, because I caught a glimpse of it in the crossfire between two enormous online travel agencies and lived to tell the story. In fact, online deals are often so elusive that you shouldn’t count on anything until you have a written confirmation.

What you see on the Internet isn’t always what you get. Who hasn’t clicked on a deal, only to be confronted with a pop-up window that says it’s sold out, but you can book at a higher price?

My adventure as a virtual war correspondent began with an e-mail from a large online travel agency. It claimed one of its competitors wasn’t playing fair. Its most incendiary accusation: that the competitor was advertising low but un-bookable airfares through Google ads. Clicking on the banner would take you to a search wizard, not the actual fare. Classic bait-and-switch.

I asked the competitor about the charges, and it quickly fired back, insisting the accusations were “false and defamatory.” When the customer clicks on the link, it noted, they would be directed to the flight listing page, which would display flights offered at that price for that route.

The competitor then launched a counter-offensive, pointing out that the online travel site leveling the accusations against it was guilty of the same thing. It sent screenshots to prove it.

What’s more, the accused online agency added, its competitor employed hard-sell tactics to persuade you to book quickly. It would tell someone who just finished a search to “Book now and don’t miss out on the price.”

As I reviewed these allegations, it struck me that the bag of tricks online agencies use to entice you into buying a seemingly inexpensive ticket, hotel room or rental car are expanding. No wonder the Transportation Department is flexing its regulatory muscle with a new proposal to add a series of new rules that, among other changes, would require better disclosure of fees and surcharges from online travel agencies.

Sure, a too-popular vacation package sold online by an airline and the day-to-day tricks online agencies use to persuade you to book a ticket are not exactly the same thing. But to travelers who often don’t know, or don’t care, about the difference between a direct booking and an agency booking, they are one and the same: frustrating.

I’ll hand it to Icelandair. Raucheisen says if Spencer’s package was completely unavailable, the airline “would have taken the Web page down.”

But is it asking too much to require that anyone selling an airline ticket, hotel room or rental car actually make good on its advertised price?

Should online prices be regulated better by the government?

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A few tips to get the best deals

Don’t be the last to know. Sign up for e-mail notifications or follow an airline or hotel on social media for real-time deals. Southwest Airlines offers an app called “Ding,” which alerts you to discounts at your home airport.

Don’t be indecisive. Discounted fares are in limited supply, and once they’re gone, there’s no bringing them back. Be prepared to take advantage of a sale as quickly as possible.

Don’t count on it until it’s confirmed. Prices can change up until the last booking screen. Your “deal” isn’t a deal until you have a confirmation number.