The Hogsmeade Post Office is a building in the village of Hogsmeade that houses at least two hundred owls so that wizards can send mail. The owls are colour-coded according to the speed of their delivery : little Scops owls, for instance, are used only for local deliveries, whereas the larger Great Greys can handle longer journeys. Owls leave the Post Office at regular intervals.
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 Invisibility Cloak to get to Hogsmeade, they point it out to him, as well as several other shops of interest, including Zonko's.