Upvote:1
In most Customs you can choose between two paths.
The first is "Nothing to declare" - that means you don't need to declare anything you bring with you, whether you bought it with you from you country, bought it in the Duty Free or it falls in some duty exempt categories.
The second is "Something to declare" - this means you need to pay custom on some of the items you brought with you.
Now some people are stopped on the nothing to declare lane for random checks and then they need to prove their items are exempt from duties. A duty free receipt would be good for this.
The government can freely decide where to place duty free zones. In my country we have an entire city that doesn't need to pay VAT, if they choose they can make an airport, even without border control, be a duty free zone.