I send a range of items that are suitable to "flat pack" in rigid cardboard mailers.  Saves loads of post $$$

There are several reasonably priced AU sellers.   From my experience, a great seller with a good product, reasonable price if buying in large quantites & excellent service who has a range of different sizes sells "SuperFlat 310x220x16mm Large Letter Size - A4 size Rigid Envelope Mailer" - if you search that title you can go to their store from there and have a look, they include info and advice re Aust Post dimensions too.