Just.go.to.the.nearest.qualifying.post.office..!
Yeah they have queues but you can time it right, or come back another time. When I went to renew about 3 weeks ago they had closed my local PO at Erasmusrand (taken over by Postnet who also wanted money and it would take 3-5days :-( ), Menlyn PO was so crowded the single queue was out the door so I looked up the next closest PO on my phone & went to Garsfontein, still only took me less than half a Sat morning to renew my discs..
What I like is they're directly connected to Natis, so if there's fines or you're late (like I was) they just slap the appropriate penalty or one time they processed my application & gave me a cert but withheld my disc 'cos there was an outstanding admission-of-guilt fine I hadn't yet paid. Direct feedback baby not waiting to hear in 5 days time etc.
Its the only function I still use the post office for, but 99% of the time it just works 100%