In Hayford Pierce's Galactic Postal Union stories, his import/export capitalist protagonist Chap Foey Rider discovers the existence of the Union when he observes that letters are delivered faster the further away you send them. So he sends a letter to Alpha Centauri and gets a response immediately.
I observed further evidence of the phenomenon last week when I mailed 4 identical envelopes on Saturday 27 November in New York to London, Colorado, Hawaii, and Hong Kong. The London, Colorado, and Hawaii envelopes were delivered on 1 December. The Hong Kong envelope was delivered on 30 November.