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I am not one for buying things in airports but recently my wife asked me to get her an alcoholic beverage while flying from the USA to Ireland (JFK-DUB). I made the purchase in JFK and was surprised to be told I had to collect it when departing for my flight as I board the plane. The sales assistant's English wasn't their best so I may have misheard and my Yorkshire twang may have thrown her, but I was somewhat surprised as to why I couldn't carry the bag myself to the plane.

Is there a law that prevents me from carrying my purchase to the plane or is this an added service I was charged, which I was duped into paying?

When boarding the plane there were several others who had to pick up their sealed duty free bags (why sealed?) so it wasn't just me. No explanation was given.

The laws around duty-free require that you not consume duty free merchandise in the country you're buying them in. If they just handed you the bag, you could then exit the terminal and go home, evading paying duty; people would buy cheap tickets for planes they had no intention of boarding, just to get a shopping discount. By holding the merchandise until you get on the plane, they prevent you from doing that. This isn't specific to the US... I've seen it in various EU countries as well, for duty-free shops located before exit checks.

I have never, ever seen a charge for the "service".

In some countries, where domestic and international flights are segregated, you buy goods in DF shops and take your purchases with you (yes, indeed, in sealed bags, it's a legal requirement very few people abide to). That's because people have no other place to go than out of the country. So there's no risk of "leakage".

US airports are not segregated. You can fly to Dublin, and at the next gate, people are boarding a flight to Chicago or Denver. That won't do. If DF purchases weren't handed over at the gate, an international traveler could pass the purchases to a domestic traveler, circumventing the system. Which is why DF shops have to make sure the goods will leave the country.

There was a related question some time ago, about buying DF good when leaving the US with a transfer within the US. The answer was, of course for the same reason, at the airport where you board the flight leaving the US, and not before.

When boarding the plane there were several others who had to pick up their sealed duty free bags (why sealed?) so it wasn't just me. No explanation was given.

This is an "Official Security Bag" for duty-free purchases.

From the CATSA (Canadian airport screening authority) webpage on Duty-free Purchases as Carry-on Baggage:

These are sealed clear plastic bags with special security features that hold goods purchased from airline or airport retailers after security screening. They are designed to make it easier for you to carry liquids, aerosols and gels through screening.

Official security bags are used in several—but not all—countries around the world. Wherever you travel, check first to make sure your bag has the security features shown in the image here – typically a checkmark and arrow in a circle as well as a red border - when you make your purchase. If it doesn’t, it will be rejected.

The idea is that if your international flight is not a direct flight to your final destination, some airports/countries may require you to re-clear security prior to boarding your final flight. (For instance, if you had a stopover in Canada, in most cases you would have to re-clear security after the Canadian passport control.)

Normally you aren't allowed to take more than 100 mL of liquid through security and normally duty-free alcohol purchases are in increments of more than 100 mL. In order to facilitate alcohol duty-free sales, airports & airport security have worked together to provide some form of secure exemption to the above rule which results in these security bags.