Simple and well tested concept. Well executed for delivering packages.
As others already mentioned it, music and sound effects would have been nice.
Did you draw all sprites yourself?
Only slightly annoying point is that the mailboxes don't close when the parcel is not 100% inside, but it's easy to restart and hit the shoot on the next go because I don't have to move my mouse. Very convenient 😉
If you work on a future version I would recommend having more mailboxes in some levels and adding some cool mechanics, like bouncing pads or teleporters.