If you want to drive across the border from South Africa into neighbouring Namibia, then you need permission from your hire car company in the form of a letter including the vehicle registration number and chassis number. 

This must be presented at the border post, and a permit will be issued which must be carried at all times while you are in Namibia.  There may be a fee payable as well. 

In terms of paperwork, you can expect to spend about 15 minutes walking from Immigration to Customs, and having your passport checked and stamped, but overall, the procedure is surprisingly painless. 

There are 2 major border posts when crossing into Namibia:

Noordoewer (open 24 hours) - when travelling from Cape Town and the Western Cape

Nakop (open 24 hours) -  when travelling from anywhere else in South Africa