You can use generic BX1 boxes which cost about 40c.  I've lodged them at various Australia Post outlets including Melbourne CBD on Elizabeth St - who you would expect would know what they are about - with not even a raised eyebrow.

Another trick is to used the assessed postage options.  The 500g flat rate is standard across Australia even if you send it next door HOWEVER if you send something bigger within the same state (and won't fit in a BX1 box), it is worth checking the Australia Post Parcel cost in the same drop down menu of the ebay label page.  Put the dimensions in + weight and it can be as little as $9.