Letter – (n1) A graphic symbol, alphabetic sign, or written character conveying information about sound in speech; (n2) A message or missive communication in writing; (n3) The strict interpretation of the literal meaning.
Missive – Sent by superior authority.
God used this word in its various forms 71 times in Scripture: 49 times to Israel in their Masoretic Text Old Testament; Twice to Israel in their Koine Greek Old Testament; Once to Israel in the transition book of Acts; 5 times to the Body of Christ in the transition book of Acts; 13 times to the Body of Christ in the Gospel of Grace; and once to Israel’s Little Flock in their tribulation book of Hebrews.
God used the word as a contrast between the ‘letter’ of the Law and ‘spirit’ of the Law. Teaching that you can know every detail and still miss the point; that you can be perfect in every aspect and still be wrong.
Not all letters carry the same weight. God records several messages in Scripture, some of which are tidings. He also records several missive communications in Scripture. And some of the letters recorded in Scripture are epistles.