I love a challenge,  I just entered the dimensions into the calculator, Sydney to Brisbane eg box 45 x 39 x 25 weighing 4.5 and it automatically comes up as Express says $52, BUT click on REGULAR post at the top, and it is only $21  😄  it may differ a little depending on the postcodes, but still less than half.

Go to your post office and get an exact price, and still consider the bag I suggested to avoid the cubing issue.