Going Postal by Terry Pratchett

Moist von Lipwig, a conman on death row, is given an ultimatum by the despot that runs the city-state of Ankh-Morpork: Be executed for his crimes, or… serve as postmaster and revive the city’s ancient, moribund postal service (???)

Hijinks ensue. Pratchett gets me laughing aloud at least once per page. Don’t let the fact that it’s part of a series dissuade you, it stands alone & has been a big hit with both the fantasy & non-fantasy readers (me!) in my life.