The only contact details I have for them are the same as on their website:
[email protected] or whatsapp +27 72 139 5903. At a minimum, ask for a breakdown of the charges and the tariff heading of the declaration.
If you are successful to challenge them, you'll be the first that I know of. They're known to charge ridiculous handling fees that increase rapidly with item value and getting hold if them is near impossible. (I've tried before and received no answer whatsoever.)
I agree with you though - a "smart switch" should be close to zero duty and just VAT on 110% of the value + the handling fee. If it was incorrectly declared as a circuit breaker, it might fall into the 15% category though. In the latter case: R162 for VAT, R162 for duty and the remainder probably probably R220 handling (+ 15% VAT = R254). I would not be surprised at all...