Some talk on the game-help channel of sending letters gave me this idea! Envelopes might make a nice addition to the game, if you ever wanted to send someone a letter and be just a bit more confident that your courier won't read it. Sure, there's nothing keeping someone from opening an envelope, but the recipient would be able to tell if it had been opened on the way - unless maybe using the thievery skill or something similar to discreetly open envelopes is a thing that is possible.
Having an envelope to open might also add that little extra bit of tension to getting various kinds of 'letters of termination', as well, perhaps...