Can the six boxes be consolidated into one box or are the destined for different addresses. And just to confirm, it's only 20kg? Is that 20Kg each box or for all combined?
I was at first going to suggest sea-freight. However, the weight is too small. Therfore if I understand the facts correctly, you may not be that bad off using Parcel Force or TNT, although TNT is a bit dearer. If it's urgent, there is Freight Packaging service where you carry the item to the airport, say on SAA and your designated recipient collects it in J-berg or Capetown...
I've used Interlink for packages to Australia and Kenya and their prices were lower, however I know that they 'sub' out the services to many different companies, so you never know who will be handling it at the other end.
Best of luck
Fr Bill