the postcsrd had a real-looking Australian postmark
So, does Warwick draw one on and just put the card in his mailbox? He didn't know John was coming. How else could he get a card with that particular postmark on it if he wasn't in Australia (and it would have come up if he'd been there)? And if it was mailed from anywhere else, it would show another postmark, probably right over the drawn one.
This makes no sense. The camera showed it in the frame, which was not really necessary--and it spoils the story.
A lot of this story was logically difficult to explain, but I didn't mind. This, I minded.