There are a number of M&G services. I know the Afton House has one as well, as I am using it for my trip in May.
The question was, will the invitation letter from the Namibian outfitter satisfy the requirements of the RSA authorities or does the letter have to come from someone in RSA?
I'm not sure what you mean by "satisfy the requirements of the RSA authorities"? If your final destination is Namibia, have the Namibia invitation letter handy just in case someone asks. You won't need one from RSA just for transiting through J'Burg. I don't remember if I needed the Namibian invitation letter or not for the SAPS permit but I can check later and let you know.
I've used the meet/greet and pre-approved SAPS service every time I've gone, and once my rifle showed up in the SAPS office I've never spent more than 5 minutes actually clearing the rifle.
