The Hogsmeade Post Office is a building in the village of Hogsmeade that houses at least two–to–three–hundred owls, ranging from Great Grey Owls to tiny Scops (the latter for "local deliveries only"), hooting down from colour-coded shelves. The shelves are colour-coded based on how quickly they will arrive at their destination.
Ron and Hermione visit it on the first time they go to Hogsmeade, and later tell Harry about it. When Harry uses the Marauder's Map and his Cloak of Invisibility to get to Hogsmeade, they point it out to him, as well as several other shops of interest, including Zonko's Joke Shop.