A single written sign, and a written message sent to one person. Both are marks made to be read by another.
Latin littera (a letter of the alphabet; in the plural, an epistle), of disputed further origin, perhaps borrowed from a Greek word for a tablet; a single written sign, and a written message, are the same word because both are marks made to be read by another.
A single mark and a message sent to one reader share a word because both are marks made with someone else's reading in mind.