A level editor??? Okay
The gameplay progression is nice. Simple at the start, but rises up in difficulty along the way. Also introduces mechanics neatly. And it's impressive how there are so many different AI movement here. Really makes thing interesting and more complex.
Maybe a speed up button would be nice to have. I can really feel how slow the game is during the level where you introduce mailmen that won't carry more than their coworkers.
Another feature I would like to be added is a restart level button. I don't really like having to manually destroy all paths if I made a huge mistake.
Theme wise, I think it has some correlation to it. Depending on how you set up your paths, it can lead to different results. Combining that with how each mailmen interact with each other and all the timing that you need to do, and you have a small chain reaction.
I like the aesthetic of the game. Pretty simple and clean.
The audio is pretty nice. Relaxing music that accompanies relaxing gameplay. SFX are pretty good as well.