Thank you to all who have replied. I am using my own packaging (boxes) I have been in touch with Australia Post and also eBay, so hopefully things will get sorted out soon. Seems it is a glitch due to the price changes. I have had no problems with printing postage labels in the past, although haven't sold since earlier in the year. They were always spot on to the cent, and then a discount to me for printing my own labels.

On the plus side, finally Australia Post has made a fixed prices for parcels using your own or their packaging, up to 5 kgs.  So now a parcel weighing 500gram to 1 kilo, will travel anywhere in Australia for $12.20.

The latest book from the Post Offices dated 30th September. 2019 is worth picking up.