You can send a 3kg Satchel anywhere in Australia for $11.70.  A 500 gm Satchel will cost $7.15 with ebay postage label a little more with Aus Post prepaid satchels.

If you are sending the parcel registered  (this is your Paypal protection) then why should your buyer pay for your protection.

Unfortunately it is against ebay Policy to ask for more money after the auction has finished so your options are:

Send the parcel at the winning price and postage.

Agree to cancel the transaction and hope you don't get a red dot.