This film was shot little by little over the course of a number of years when the different actors were available. In real life several of the actors never met while filming.
The movie is a fictional mockumentary partially based on the childhood of writer/director Benjamin Ironside Koppin.
Director Benjamin Ironside Koppin and his brother Matthew John Koppin created Secret Mailman as kids. This movie is a fictional retelling and continuation of those stories including actor Halynn Batterman playing their fictional sister. In real life they don't have an additional sibling and Matthew John Koppin is very much alive.