Duty free is a tax rule, not a discount

This is written to be used rather than admired. Each section below is a decision about duty-free shopping, and each one has a default.

What the exemption actually removes

Duty free means goods sold without the excise duty, and sometimes the sales tax, that would apply within a tax territory. The exemption applies to particular categories, most commonly alcohol, tobacco and fragrance, where duty is a large share of the shelf price. For goods where duty is small, such as most electronics and clothing, removing it changes almost nothing about the price.

The shop still sets its own margin, and airside retail carries high rent, high staff costs and a captive audience. A price can therefore be genuinely duty free and simultaneously higher than the same item in an ordinary shop.

The allowance is at the other end

What you may buy is limited by the shop, while what you may bring in is limited by the country you are entering. Those allowances are set per country, differ by product, and often differ again depending on where you have travelled from.

Exceeding an allowance means declaring the goods and paying duty on arrival rather than being turned away at the border. Allowances are usually personal, and pooling them across a family is permitted in some places and not in others. Because this varies so much, the destination customs authority is the only reliable source for the actual numbers.

Where the shop is duty paid anyway

Travel within a single customs or tax union is often not duty free, even though the shop sits past the security checkpoint. In those cases the airside shop is simply a shop with a captive market and all the usual taxes included in the price. Signage sometimes distinguishes the two situations and sometimes does not, which is why comparing prices beats reading labels.

Domestic flights within one country are almost never duty free, for exactly the same reason. The useful habit is checking a price against what you would pay at home rather than against a claimed saving.

Liquids, sealed bags and connections

Liquids bought airside are usually sealed in a tamper-evident bag with the receipt, which allows them through later screening in many places. That arrangement is not universal, and some countries will not accept a bag sealed elsewhere, removing the contents at a connection. Buying liquids before a connecting flight therefore carries a risk that buying them on the final leg does not.

The working rule: keeping the receipt visible inside the sealed bag is what makes the system work on the occasions when it works at all.

Anything expensive is safer bought at the last airport before your destination rather than the first one.

Arrivals shops and pre-ordering

Some airports operate duty free shops after arrival, which removes the problem of carrying bottles through connections entirely. Pre-ordering online for collection at the gate or on board is common and occasionally cheaper than the shelf price.

Nine times in ten, onboard sales are usually the most expensive option, because the range is small and the audience has nowhere else to go. Currency matters, since airside shops often quote in several currencies at a rate they choose themselves. Paying in the currency the shop is priced in generally avoids a poor conversion being applied at the till.

Border and customs rules are national and are the ones worth double-checking.

When it is genuinely worth it

Spirits and tobacco bound for high-duty countries are the clearest case, because duty dominates the price at home. Regional specialities that are hard to buy elsewhere can be worth it regardless of any tax argument.

Anything with a published international price, such as electronics, is easy to check and rarely turns out to be a bargain. Cosmetics and fragrance sit somewhere in between, with real discounts on some lines and none at all on others. The test is always a price you already know, and a phone is enough to establish it before you reach the till.

The takeaway

Compare against a price you already know, and buy the high-duty items rather than everything on the shelf.

The trip goes better when the bag is boring.