The Dead Letter Office by Gillian St. Kevern

Look what the mailman dragged in…

Jasper Carruthers has turned deciphering smudged addresses and avoiding conflict into a fine art. A crate from Egypt contains a problem he cannot return to sender: a mummified cat sought by a desperate thief. Failure to deliver the cat will give the Postmaster General—Jasper’s vengeful son—the excuse he needs to oust Jasper from the postal service.
Jasper’s attempts to deliver the package attract the interest of Captain Candy, an insufferable bore under the mistaken impression that Jasper tolerates him. Even worse: the cat does not seem to realise she’s dead. Jasper’s not sure if he needs an Egyptologist or an exorcist. There’s only one thing he’s certain of: he needs help.
Forced to trust Candy with his secret, Jasper may at last have found something worth fighting for—but can he deliver the package before the cat lets herself out of the bag?