Ebay have a 10% discount at the moment on 500g assessed parcels.  So when you do the postage label in ebay you select Australia Post Parcel Post Parcel and put in the dimensions and weight at under whatever your item is at under 500g, the postage should be $7.20.  Ebay give you a 10% refund which you can see in your invoice about a day later.  So the postage is only $6.48 or something (I might be a few cents out).  The promotion runs until 31 December.

There was another thread on this about packaging - I'll get in trouble for this by other posters but I use the ebay satchels and haven't had a problem - I have been lodging them at the PO regularly for the past few months and no-one has ever batted an eye as they happily scan them.  But other sellers here feel you should turn the bag inside out and stick the label on.  Or you can recycle old tough bags etc which I've done sometimes as well.  Up to you.