Excellent cosy little incremental game. Really good job with the introduction of mechanics and eventual automation, lovely polish, and a great overall aesthetic. Regarding some of the things others have brought up:
Even for someone not-colourblind, some of the colours are quite close to each other. Something you could try is giving them different saturations, or adding a symbol or small particle effect along with the colour to distinguish them better.
For balancing the desk, I think the simplest solution would just be to mandate that you can't upgrade to the next desk until you purchase X number of tools ("the Postmaster wants to make sure you're doing your job properly, you're not just in it for the stamps!"). If you really wanted to streamline it, you could even prevent certain malfunctions from appearing until you have a certain desk level (maybe that's a stated downside of upgrading the desk?).
On a technical level, the only thing that bugged me was how small the screen was, even if fullscreen'd. There's a lot of unused space in the window you could've used!
But for what this is, it's great. For taking it further, I'd love to see more interactions in the parallel universe. What does the parallel Postmaster make of this? Could they give us some additional tasks? Or maybe you have to deal with a mail sack on both sides? I think there's lots of potential there!