Key material distribution is a completely separate problem from authentication of a message.
Even if you use an authenticated encryption mode, you still have the problem of "how does this symmetric key get shared with only the intended parties?".
No they are sent by snailmail.
Don't you offload the authentication to the postal service in this case?
Any offline method works. Transfer by handshake, travel with it in your bag from the main office, or injected as a chip under your skin since birth, any such method. :)