I don't really like how the time travel in this works. He sends a letter to the past and changes to the timeline slowly integrate into his memory.

I'd have preferred it if the world is destroyed the moment he places a letter in the box and his consciousness returned to the moment the time the letter was sent to, at least in the case the letter is sent to the past. If he sends the letter to a time before his birth, then his consciousness returning to his infant self... more>> would also work.

The way it stands now, it's difficult to apply any significance to sending mail and the narrative is not very cohesive or immersive. <<less