Swedes also buy from Denmark and Danes buy from Germany. The entire thing is really fascinating.
- Pennsylvania has relatively strict laws regarding alcohol, so just over the Delaware border you'll find a lot of these "border shops" (although I haven't heard that word in the US); - my wife was born in a dry county in Arkansas. Just over the county line (which happens to be the state border with Oklahoma) there are drive-through liquor stores. You can go, get your booze, and turn right around and go home.
This kind of airline price discrimination, however, is not legal within the EU.