I have been wrongly charged postage costs on an item I payed to post,

[musogreg70]

I bumped the ‘buy postage’ on the app, despite having posted the item already and payed for it.

So EBay  instantly provided the wrong tracking number, and took $. 
A two step system would be more useful than someone going to check postage rates etc and being instantly debited. Not cool. 

[padi*0409]

Go to the link below and scroll to the bottom for the "contact us" button, ask to speak to a supervisor and go from there.

You'll probably have to wait a while so that they can see the label hasn't been used.

https://www.ebay.com.au/help/selling/shipping-items/labels-addressing-packaging-tips?id=4085

[davewil1964]

Why check postage when you've already posted?

At worst you could set up a listing using calculated postage and see what that says. Or not bother with eBay postage as it is never cheaper than Australia Post.