I believe the reason you sometimes get charged customs, and sometimes not, is the same reason you sometimes receive your goods in a timely fashion, and sometimes never receive your goods at all. Customs is one of the worst managed departments in the government at the moment. Which is sad, because the revenue side of SARS is one of the most efficient.
I had the 'privelege' of visiting the inner sanctums of the border police office at SARS customs at OR Tambo a little while back. What an absolute mess. Uncategorised piles of stuff lying everywhere: from electronic goods and cameras to clothes, shoes, books - scattered haphazardly across the floor in plastic bags and loose piles. And the filing was a joke: random piles of documentation on desks, in drawers, in cupboards. I was inspecting a shipment of computer monitors my firm had ordered: it took the customs official 40 minutes of searching through these random piles for our paperwork before she eventually gave up and just started a new file!
We often blame the Post Office for a lot of our orders that go missing, but I can tell you that Customs is at least half as much to blame. One of those monitors in our shipment was mysteriously 'lost' by the border police.
I've also had some lighting equipment (stands and brollies) that was kept by Customs for four months! I'd figured it had been lost in the post and had already begun negotiating with the US supplier for a new delivery when the heavily damaged box, wrapped in SARS Customs tape to keep it together - finally showed up. Luckily none of the contents were missing or damaged. After seeing the disorganised mess in their offices, it makes perfect sense to me why it could take four months for a shipment to go through that process.
Anyway, none of this has deterred me from buying photo equipment from overseas. I've been lucky mostly - never had anything really valuable go missing/undelivered.
As an aside, when you order smallish parts from Hong Kong off eBay they usually post it in plain manilla envelopes, and those never get inspected or charged customs/VAT.