Green’s Dictionary of Slang

(S.Afr. Und.) a middleman in the illicit liquor trade.

1950		Cape Times Weekend Mag. 17 June 5: As soon as the bottle store opens, the mailer is there. He gets his regulation two bottles and takes this to the shebeen. Then he goes to another bottle-store for a further two bottles. And so he goes on the whole day [DSAE].
1962		T. Hopkinson In the Fiery Continent 101: These last were ‘mailers’, purveyors of illicit liquor. They buy drink, chiefly brandy and beer, at liquor stores [...] and turn it over to the shebeen ‘queens’.
1977		D. Muller Whitey 32: By this time the legal bottle-stores would be open, and the ‘mailers’ — the runners from shebeens big and small — would be at the counters, buying supplies for the long-week-end-thirst.
1986		D. Case Love David 110: ‘Yes, Oupa,’ David explained. ‘I was working with a mailer. The day that they caught us, I had a few parcels on me.’ [DSAE].