First, I noticed for ZA that their ECONOMY and COMPACT classes are by and large the same vehicle, which is only SLIGHTLY larger than their MINI class (not the actual Mini car, the MINI class).

Second, I am wondering is whether you have any coupons, especially a car upgrade coupon.

Third, I also noticed that the time of day for pickup in the Joberg area will effect the rates for certain days, have you fooled around with time of day or varied your pickup days forward and backward by a day or two?

Fourth, I checked Joberg, but not Capetown or Durban, so where you are flying into could make a big difference in the rates.

Fifth, have you tried both 30 day rentals (the longest possible to reserve online, so even if you will be there for 31 days, you will need to call) and a succession of 7 day rentals to see if there is an unseen "surplus" charge for longer rentals?

Come back with more information and I will try to dig up more assistance for you.