One of the ways to mail sheets of acid, in the days before more advanced chemical inventions, was to hide it between two sheets of carbon paper, keeping x-ray machines from seeing the perforations (which was how The Man could tell the difference between The Rainbow Family and The Philatelic Family).
With the advent of NCR paper, however, the use of carbon paper is now outdated, making the above method as suspicious as mailing home potpourri gifts from Amsterdam.