Do this. If I recall correctly electronics goods are customs free. As others mentioned you are still required to pay VAT. The way this is usually calculated is they look at the value of the items listed on the package (if none is given I think they make something up), they multiply this by some dollar:rand exchange that isn't particularly favourable and then multiply it by 14%.
On top of this they charge a handling fee which is usually only about R20 (can't remember exactly).
When I received mine I didn't have to pay any taxes, it seems like luck of the draw a bit. But I advise finding out what values they used to calculate how much you owe, and deal with it from there.
wrt to post/delivery. No one paid postage on their orders from RS for delivery of the raspberry pi. It's all supposed to be included. Originally they specified that it would be delivered and many of the orders in the first 4 months were delivered, they have since decided it's better (i assume just cheaper) to post the packages. This is their decision, and although I don't think it's right, they're still getting the package to you for free.