Curiously enough, while the island is served by Britain's domestic postal system, mail sent there is first sent to South Africa, a foreign country, where it is placed aboard a fishing boat, the total journey taking up to a month. (Tristan Da Cunha is not reachable by air, for lack of an airstrip.) Which means that either (a) it is one of very few British domestic destinations to which mail has to go through customs, or (b) this is a loophole which allows parcels to travel between Britain and South Africa without customs clearance.