Unless a firmware update changed things, the D50's internal formatting routine could only handle up to 2 GB SD cards. You could use the rarer 4GB SD cards if it is formatted outside the camera. When it was released in 2005, 2GB was quite a lot.
Since you're buying an SD card to mail stuff to Australia, I don't quite see why it matters whether it works on a D50: you're expecting the other side to mail the card(s) back?
The local convenience store has 64 GB cards for around $28, $3 more than 32 GB.