In Durban it will have to go through customs procedures and other formalities need to be arranged. Try to keep the commercial value at a minimum, 200-500 USD. You can also try and get an air quote, to compare if it is equal cost.

Finally, the other option is postal service, and again, keep the commercial cost at a minimum.

One suggestion I have is that you mentioned duffel bags, these are not sturdy enough for such a long time and distance traveled as there will be too much handling involved. We all know how luggage is handled, don't get me going on cargo.