We don't sell/ship time critical items....

But if we did and a buyer purchased one without choosing the express option as in your case, then I think I would have been inclined to send them a friendly message just to say that as the item is time critical you would strongly recommend considering an upgrade to express post for $xx because Australia Post do not always deliver in the timeframe they say they do. Then if they chose to either ignore or decline your offer then they have zip to complain about, having been forewarned.