The airport operator for South Africa has a pretty decent Web site; I just don’t have anything to validate the precise locations of the post offices.
On the “shops and facilities” page they state the following:
There is a post office at the international arrivals terminal, and you will find Postnet on the shopping level of the domestic terminal.
Unfortunately I couldn’t get the maps to load, so I can’t verify the information.
Last updated 19 July 2017