I suggest using a Flat Rate box, it's 18$ USD for the small Flat Rate Box which is all I ever use for shipping, and have shipped a few times to AUS, CAN and other countries. Priority mail doesn't give you tracking but, Express Mail (Flat Rate) does, but is more expensive.
As others have expressed you can use usps.com and the AUS postal code to get a pretty decent estimate. It might be cheaper to use your own packaging if it is light, which I guess it is for note cards. My items are heavy to its the best method for me, but I stand by it! !