Shannon, Ireland, population 10,000, is a great jumping-off point for travelers heading to Limerick or the Cliffs of Moher. It’s also the birthplace of a $68.6 billion industry: duty-free shops. Shannon Airport opened the world’s first such store in 1947, offering people passing through the western part of the country the opportunity to shop without paying taxes before they left.

“To spawn what has become a global, multibillion-dollar industry is something that we’re very proud of,” says Darren Smyth, head of commercial operations at Shannon Airport. “Everybody the world over likes to feel like they’re getting good value.”

What Shannon started became an international phenomenon. Anyone who’s traveled internationally — by plane, cruise, ferry or overland — has probably encountered duty-free shopping. While many travelers are familiar with the concept, it can still be confusing in terms of value. You know you’re not paying local tax on items such as cosmetics, cigarettes, sunglasses and spirits, but is buying goods here really a better deal than elsewhere?

Depends on what you consider a deal. When the Shannon Airport launched duty-free, a lower price thanks to the nixed tax was the key selling point. Today, duty-free shopping’s appeal is more complex. Now that we can comparison-shop for deals online for just about anything, duty-free shops had to come up with new ways to entice customers. Here’s what you need to know before you head to the airport.

The real savings are in alcohol.

The best deals at duty-free will depend on the taxes where you live; the higher the tax on a product, the better the deal at duty-free. Around the world, “sin taxes” tend to be a common practice, making “sinful” products your best buy.

“The heaviest-taxed areas of all our lives are things like liquor and tobacco products,” says Tony Richardson, founder of Duty Free Hunter, an online shopping guide for travelers. “They naturally are going to have the biggest savings, because you’re escaping either duty or sales tax, or both, depending on the regime in a particular country.”

Sales of booze at duty-free stores are so strong that companies will send brand ambassadors to airports to hold tastings and food-pairing sessions to get more eyes on their merch. Mumbai Duty Free, at Chhatrapati Shivaji International Airport, even hosts an annual whiskey week.

“Every shop now, they are trying to create a bit of an experience as well,” Richardson says. “What we’ve seen in the industry is the retailers and the big brands trying to create a sort of unmissable factor.”

If you’re in the market for makeup, look for freebies.

Cosmetics brands sweeten deals by creating special travel-size versions of popular items or offering freebies with your purchase as a perk, Smyth says. Your standard-size lipstick may cost about what you’d pay at home, but at duty-free, the brand may throw in an extra gift.

“It’s useful in terms of maximizing the value at a duty-free,” Smyth says.

The sales associates will be the most up-to-date on brand promotions, so ask them about potential gifts with purchase before you start browsing. You can also go online to Duty Free Hunter or, for travelers going to the U.K. or Germany, World Duty Free, to find news on travel retail sales.

Find items you can’t buy anywhere else, especially spirits.

For some brands, making limited-edition items available only in duty-free stores is a way to reel in new and existing customers.

“What you’ve seen more and more is the use of exclusives,” Richardson says. “There will be certain perfumes or drinks brands in particular that you can only get when you travel. Johnnie Walker will create a whiskey that is only available in certain airports, so that way they’re giving travelers a reason to make sure they go in the shop.”

Spirits brands, for example, may release multi-bottle series for these stores to tempt spirits connoisseurs. Like playing Pokémon, you may be compelled to try to catch them all.

“The idea for now is that we need to offer passengers something different that you can’t ordinarily get,” says Manjot Riyait, head of marketing for Global Travel Retail at William Grant & Sons Global Brands, the independent spirits company responsible for brands such as Glenfiddich, Balvenie and Hendrick’s gin. For collectors who already have staples from a spirits company’s core range, scoring a bottle at duty-free you can’t get at liquor stores, such as the Balvenie Triple Cask range, can be a huge win.

Booze companies aren’t the only ones to make moves like this. Cosmetics companies and fashion labels are also known to release travel-retail exclusives. For example, the French fashion brand Chloé just released a four-piece capsule collection for DFS, a luxury travel retailer with duty-free shops in 11 major airports.

Candy is a nice present, but you won’t save cash on it.

Even though sweets make up a large chunk of real estate in duty-free stores, it’s not there because it’s a fantastic deal only found past the security checkpoint.

“With confectionery, there isn’t a huge saving, because the price points are much smaller,” Richardson says.

He says prices on sweets will vary where you go around the world due to local market factors, so you may be able to find Toblerone cheaper than you would at home. The biggest perk of buying desserts at duty-free is that it’s an easy last-minute gift idea. Many candy brands will stock the shops with packages that are ready to be presented as gifts.

Take advantage of international brands you can’t find at home.

Duty-free stores study their customer base’s demographics and fill their shelves with items curated to appeal to groups traveling through certain airports. That means fliers may encounter products they wouldn’t normally at their hometown mall.

“You might see Asian brands that you can’t get anywhere else in America, but you can get them at the San Francisco airport, because there’s a demographic traveling through there that they need to service,” Richardson says.

Skip the sunglasses.

As with chocolates and candy, the most persuasive case for shopping for sunglasses at the airport is the convenience. If you forgot your Ray-Bans at home while en route to Cancun, at least you can get some before you hop on the plane. Richardson isn’t sold on buying shades at duty-free for cost purposes, though, saying there’s no massive bargain.

“You may get exclusive lines, but they’ll be minimal in terms of saving,” he says. “If you dig around on the Internet, I would think you’d find them for the same sort of price.”

Research, because some items may not be cheaper at duty-free at all.

Retailers may offer higher discounts in some markets than in others. For example, a bottle of Lagavulin 16-year-old Scotch whisky at Singapore’s Changi Airport runs for the equivalent of about $90 U.S., while it can be had for only $60 U.S. at London Heathrow. Sometimes prices vary within a single airport. According to findings from the Points Guy’s research, duty-free prices may even differ from terminal to terminal. If you have time before your flight, it may be in your best interest to compare prices among different concourses. But calculating whether an item at duty-free is less expensive than it is elsewhere is a complicated dance.

“Trying to compare savings between, say, Singapore and New York is near impossible, because there are so many factors at play,” Richardson says. “It has so much to do with where you’re shopping in the world.”

If you have brands in mind you’re hoping to shop for at the airport, Smyth recommends taking the time to note how much you normally pay for those brands before you leave for your trip.

“Do research and give yourself time to understand what the offer is. Then you can maximize the value,” Smyth says.

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