There really is an island called Isö (or Ice Island in English). In summer the island has a population of some 20 people and I suppose it is uninhabited during the rest of the year.
Isö is a tiny island located in the Baltic Sea some 45 kilometres from the City of Norrköping on the East Coast of Sweden.
bron: StampsNewsOnline.net issue 37 - januari 3, 2014
article: "Cinderella Time: Isö—the truth about a local post" By Christer Brunström