I need to design a system for transporting a secret document from an individual in one office comple

I need to design a system for transporting a secret document from an individual in one office complex in one city to another individual in an office complex in another city. A public carrier is to be used. In other words, individual A cannot hand-deliver to Individual B. Also, an acknowledgement in the form of a receipt is to be returned to the sender. A provision should be made so that if the document must be sent in pieces, it can be assembled in the correct order.