Is it not the case in Australia that you can mail a letter by simply handing it to the mail carrier at your door (or even putting it in the box, though that's iffier)?

Um, that's not a thing in Australia.

Seriously, not a thing.

Also, you'd have to camp out at your letterbox [which is some distance from your front door, and usually has no shade] all day to catch the postie - they zoom along the footpath on motorbikes and they only pause ***even briefly*** if they have mail for a letterbox.