Oddly (Odd) Boxes

Oddly Boxes, interchangeably referred to as Odd Boxes, are the mail system of Poorthwart. They appear to be free-of-charge to use, and can instantaneously deliver any letter or parcel to any individual, no matter where they might be. It is implied that they can do this outside even of Poorthwart's physical realm, and into alternate universes or hidden realms.

Appearance
It is unclear if every Oddly Box has a different shape, since they can vary widely in appearance, but boxes resembling one another are clearly shown in different locations. They are, however, always inscribed with the word 'Post'.