Not only did the officer most likely violate some kinda privacy law by using his means of employment to gain very personal information he most definitely committed a felony when he "tampered" with your mail by placing an unstamped letter in their that obviously wasn't delivered by a federal employee with the rights to do so (mail carrier).

In short, fuck him and you by no means acted out line. Sure, it wasn't necessary that you yell that out but I woulda felt the same way and probably done somethin similar...whatever light one up.