Penny Yim-Barbieri, 47, a freelance translator based in Las Vegas, didn’t have the Cook Islands on her travel radar when she got an email alert from Scott’s Cheap Flights about round-trip tickets to Rarotonga from Los Angeles at $495. She and her husband jumped on the deal and soon found themselves scuba diving in the South Pacific with humpback whales.

“We’re divers and hikers, so we have to save on flights so we can do other stuff,” said Ms. Yim-Barbieri, who estimates she’s purchased about 30 bargain international flights found by Scott’s in the past two years.

When it comes to self-booking, it’s a golden age for price alerts, which have expanded from sale emails sent by airlines, travel agencies and other operators to fare predictions and deal disseminators, in both free and paid forms, and arriving by email, smartphone texts or push notifications.

“There is an increase in flight price-tracking capabilities, largely because of the ability of these companies to use machine learning to more accurately gauge whether a fare is a deal, and whether it is likely to go up or down,” said Maggie Rauch, the senior director of research at Phocuswright , a travel industry research firm.