Trusted? When I rented from Hertz in CPT (for the reverse of what you're doing), when I wanted to decline collision/theft coverage because my credit card covers it, they asked if I had a letter from my credit card company to prove it. When I said I didn't (I've never heard of such a thing), they demanded to put a hold on my card for the full value of the car I was renting! (Which worked but then caused a fraud alert on my card which took a long trans-Atlantic phone call to resolve.) And then at Hertz PLZ where I returned the car (on a bright sunny day), they rattled me over a tiny scratch on the back of the driver's side mirror, and a scratch on the wheel cover, both of which would have been hard-to-impossible to see when I was picking the car up at night in a dimly lit lot. In the end, they ended up charging me about double of what the Hertz estimate was, but if that settles it, that sounds simpler than arguing about it.

So at the very least, consider Avis instead of Hertz and/or consider buying full coverage (including excess at the counter). On a 5-day rental at JNB, Avis charges me only $83 to add full coverage (including excess, including tire & windshield), and that seemed a much better deal to me than risking being charged hundreds for tiny "wear and tear" kinds of things, which seems to be a common thing in South Africa.

Meanwhile, for discount codes, have you tried typical airline discount codes? For Avis (and in some cases Budget), you can find them through here:
in that the link for each airline's promotion typically also gives that airline's discount code.

Just because an independent car rental location is affiliated with someone like Hertz, that doesn't mean they can be "trusted" IMHO the way a corporate Hertz location might be able to be "trusted".