So the seller is actually registered in Australia on their feedback page? (rather than being registered in China and saying the item is in Australia?

If it is not a pick up only item (which often show as free postage) 

And you are not using the app (which often shows as things being 'free postage when they are not)

It could be 

a) A new seller not sure of how selling works

b)the seller made a mistake by not including the postage

There are a number of reasons