Thank you for taking the time to play my game, some good points to think on. Didn't think to add a second post office, but the van might be a good way to move around a source of mail to save on multiple journeys while also being more immersed in the role. Adding text to show that the number is what is being held is also a good idea. I was going to add an info page on the main menu but it felt a bit cluttered so didn't want more buttons. There are also other things around town that are not mentioned in the game. There are upgrades scattered around town to increase mail bag size and temporarily slow down time.
As for the AI, yeah they can be annoying. They kept getting in my way while testing, luckily you can jump over them or just step over the kittens without jumping. Their AI is definitely high priority for improvement, making it so they rarely go into gardens and have more of a walking down the street kinda feel to them.
In regards to the FPS, before submission I sent a copy of the game to my friend and my brother and on the three machines it was tested on we had no issues, but I guess in the grand scheme of things three machines is not many so if FPS is a major concern for multiple people that would also be a high priority for smoothing that out, could look into adding graphics settings to tone down shadows, or even reduce AI count, there are 100 cats walking around. I might have gone a bit overboard with the cat count but I wanted to make the town feel alive.
A mouse mode does sound fun, could have it take place in a back garden or by the farm so occasionally a cat would walk past and cast a mega shadow.