I must admit that these are a particular favourite of mine. There are twelve basic sets and were issued from 1914/72, a period that spans the Union and Republic periods. The printing of them ceased in 1978 and their use came to an end in the early 1980s. There is much that could be written about Postage Dues. As a subject in their own right they have as much philatelic interest as the postage stamps. For example during WWII when the regular postage stamps were dramatically reduced in size, resulting in the Bantam War Effort issue, the same step was taken with the dues, hence we have the Bantam Postage Dues of 1943/4. They also have the same changes of watermark as the postage stamps and later, the perforation change from Perf 15×14 to Perf 14.
From a collecting point of view, look out for the varieties, the early issues used in S.W.A. etc., the later issues in cylinder blocks and the reprints in different shades. The 4c on watermark RSA paper with Afrikaans at top, from a 1969 reprint (SG D62c) is amongst the rarest modern postage due of any country. This stamp is seldom offered and regularly out of stock. The collecting of postage dues on cover would make an article in its own right. Some values are found used on cover with ease, other values are truly rare on cover and highly prized by their owners! Stamps from the last set can be tricky to find on cover, often postage stamps can be seen used as dues instead. The use of postage dues dwindled in the late 1970s, being replaced by a card instructing the would-be recipient of the letter to collect it at the counter and paying the charge then.