In the movie Murder by Death, a parody murder mystery by Neil Simon, the detectives show up at the house as though invited for a murder mystery weekend, a la "And Then There Were None".
The characters play roles parodying literary and film detectives Hercule Poirot, Miss Marple, Nick and Nora Charles, Sam Spade, and Charlie Chan, and their familiar attendants. For example, the Charles[tons] were accompanied by their non-famous dog "Myron".
Early in the film, the party's host, Lionel Twain (played by Truman Capote), handwrites invitations to each, but through his orders to the blind butler, ends up getting them mailed without stamps. Additionally, the address within is stated incorrectly.
How were the invitees to deduce where and when to arrive? I'm hoping it is not dependent on the practice of postage-on-delivery, which is an outdated practice, but generally used to communicate free with businesses rather than private individuals. Also, I'm not sure postage-on-delivery would have worked for the Euro addresses to Marbles (Miss Marple in England) or Perrier (Poirot in Brussels).
I also don't wish to base this on a mundane standing order by each to accept all mail, even postage due, as prospective detection business offers.