I mean, you can put a real phone in a box and film it but it's not foolproof as you can just open the box again and ship something else. it's not an easy problem to solve. especially with different jurisdictions in different countries.
Even if you had a film of the box being delivered to the post office you could have a corrupt post office friend or a post office worker that is scamming things on their end by opening packages and replacing the contents.
So, you (the scammer) does this but leaves the bottom of the box open, and then remove the item, and glue the box back together, with a brick inside...
This is the same (sort of) reason that mailing yourself something is not proof of anything, as you can mail yourself a bunch of empty, unsealed, envelopes, then seal them with whatever inside them at some time in the future.
100%? No. But I was sure to include his USPS badge in the photo, as well as the address on the box, and the background of the behind-the-counter post office. I figure it raises the difficulty of faking it substantially.
Even legal process (especially civil legal process) doesn't rely on 100% proof to reach a result; it would be unreasonable for any dispute resolution procedure to do so.
Even if you had a film of the box being delivered to the post office you could have a corrupt post office friend or a post office worker that is scamming things on their end by opening packages and replacing the contents.