Ok. I contacted them again on a different Id. For what it's worth (probably not much)  this is what I was told:

'The issue you are facing right now is related to the introduction of discounted Au post rates since 7 Nov,2025. A temporary tech issue is causing additional postage costs to appear for the sellers in eBay.'

And

'The postage service not showing is also a technical issue and has already highlighted by the team.'