So this doesn't really answer the question, but I shipped a single box of the Rainbow Rereleases to Australia and it was about $17. Surprisingly, it arrived in less than 10 days. I would imagine a padded envelope would be under $10
Depends on how you want to ship them too. What I do is normally go onto the big shipping websites and guesstimate the weight. 3 ponies usually run around 1-3 lbs depending on how you package them. Did you get a postal code of where they are going in AUS?