Why are you not using Global Priority? I use it and it is 4-6 days and hardly any price difference from air letter. For the economy rate I would not ever use that, it can be out there forever.
I shipped to Germany and Canada last week and both arrived in 5 days, it was about 2lb 8oz, cost around $14 dollars. Use Variable Rate Global Priority. I checked, Australia would have been $13.75 for that weight.