The 2010-2011 Mawson hut restoration season at Cape Denison was the first one where they operated a post office. Appears the main reason for doing it was that it has become a tourist stop recently, as it is quite a historic site in the Antarctic. Even running a post office in a remote Antarctic location is challenging, as they do not have central heat like the more permanent bases. Since it is not heated they had trouble with the postmark ink sticking in the cold temperatures. Certainly, not something I ever thought about being a problem.
This article mentions that the first season of the Post Office postal cachets will be available on the Mawson's Huts Foundation website. The link to the Foundations online shop is here, but they are currently not available. I will try to remember to update when they do become available, as I know a lot of my readers are wonder about obtaining Antarctic Postcards and postmarks.