Letters, cards, and parcels are sorted and delivered by the Netherworld Post Office. Sending mail around the world is easy enough: every ocean, river, lake, stream, island, continent, swamp, lagoon (you get the idea) is connected to every other by land or water, but how do you send mail to the moon? And beyond?
Pneumatic space tubes and gigantic astral bats.
Given their stable rotations, the Netherworld Post is able to use long range pneumatic tubes on the earth’s and the moon’s surfaces to shoot capsules back and forth. Like clockwork, locations align and tubes fire or receive deliveries that are then gathered and emptied for content sorting and distribution.
Misfires are somewhat common as deliveries can be blown off course by cosmic rays or stray doomsday device explosions — but capsules are never lost. They fall into rotation around their respective target and are scooped up by celestial creatures on their way back from very (very) long distance dispatches.
Recoveries and deep space deliveries are handled by aether bats — giant monsters that thrive in the abyss. They eat small asteroids, absorbing the minerals and ice, on their journeys to the stars. Speed is largely determined by the size of their last meal, so many carry snack packs of metals to help when they start to slow down.
Aether bats are typically the size of gray whales on earth and can fly at warp speeds far faster than light. They communicate via a form of telepathy and tremendously enjoy stories. While very little of their lifestyles are known, they are incredibly loyal to the Netherworld Post Office and have never failed to make their promised delivery.