Build a monorail through all of a building with QR codes in place for each room and area. Then put a robotic train in it that can carry mail and other items from one place in the building to another.
It the track is large and complex enough you could have multiple robots zipping around on the rail.
In rooms with a specific place to put deliveries a separate free-standing robot could receive the item from the rail robot, carry it to the place, then return to wait for the next delivery from the rail robot.
