Suppose the "letter" in "French letter" has nothing to do with written correspondence at all? There was a time when the word "let" was a synonym of hinderance in English -- a few years back -- and one wonders if the hinderance which the condom posed both to sperm getting out and infection getting in might have led to its being called a "letter". I have NO evidence for this whatsoever; it was only a wild-eyed etymoligical guess.