I agree with Padi, sometimes we forget that you can still use a box but inside the satchel, getting the cheaper rate.

Also,, using the ebay labels with the flat rate for boxes, you can still pack quite cheaply.

e.g. the Bx1 postage is only $7.40 plus the cost of an Aust Post box

or the Bx2 postage is $12.05 plus the cost of an Aust Post box

and those rates will cover anywhere in Australia.    Usually much cheaper than calculated postage if you have to send some distance.