It's definitely worth trying to sign up to the US store. There's barely any difference in the ZA Kindle price & the prices you get at stores but it's enormously lower on the US store. The difference in content is also stark, compared to the US store, the ZA store feels like a corner stall.
It's pretty easy to sign up as being the US, just put in a US billing address. Mine is still the HQ of one of my previous employers. The ZA card works fine.
The only slight hiccup I've had is it once said that I hadn't purchased a book *in* the US for a while, got around that by buying a book while connected to a US proxy (got a free trial, changed the WiFi settings on the device) and it hasn't asked me since. That was well over a year ago.