If possible, use pre-paid satchels, it makes life easier for you as you can charge a flat rate.

You can calculate postage rates for larger parcels at the Australia Post website. On the left-hand side of their home page you can click on the link "Calculate Postage Rates", then click on "Parcels within Australia". You can then enter the postcodes, weight and dimensions and you will be given a postage quote.

Good luck