This is how duty free shopping happened in the past (1980s).
There were no duty free shops in the airport. Shops were located in a building in Kollupitiya (this is now the Senkadagala Finance building).
Those who came from overseas had to go there for duty free shopping within 3days of arrival with the passport. My dad used to do that.
It was very convenient for people living in Colombo and south.