Don't forget you have to make the export from USA as well as the import to South Africa.
There are export rules too.
First you need to classify your cars in the customs tariff - cars generally come under chapter 8703. You need to look at the US tariff for export - follow this link http://hts.usitc.gov/ and you need to look at the South African tariff for import - follow this link http://www.sars.gov.za/AllDocs/LegalDoclib/SCEA1964/LAPD-LPrim-Tariff-2012-04%20-%20Schedule%20No%201%20Part%201%20Chapters%201%20to%2099.pdf
It looks as if there is no import duty for cars under this classification into South Africa but there will still be VAT and excise duty
Value added tax (VAT)
Value added tax is levied at a rate of 14% of the dutiable value increased by 10% plus all non rebated duties.
Excise duty (EXC)
Goods falling under this subheading are subject to an excise duty rate of [(0.00003 x dutiable value increased by 15% plus non rebated customs duties) – 0.75]%, but not more than 25% of the dutiable value increased by 15% plus non rebated customs duties.
You will then need to deal with the issue of valuation - there are specific customs rules concerning how to value an item. If you are making a sale at the time of export it would be the sale price but if you are not selling the car at that point you need to follow the rules to arrive at an acceptable value.
There is also a potential issue with any fuel that may be in the petrol/diesel tank at the time of shipment as this will cause the shipment to be classified as hazardous.
I suggest you research a freight agent in the USA who is experienced with moving cars around the world - you could try Ferrari Express. Discuss your requirements with them and then get several other quotes.
I hope this helps. If you would like to discuss this further please don't hesitate to contact me either through Open to Export or direct.