a tamper proof bag or envelop usually comprise of a resistant material with the flap or opening permanently closed [1]. Some mechanism is in place to validate that the purpose opening is permanently stained upon opening. However, this will not protect the content from being accessed.
My idea is to use a self destructive envelope that will render the document unusable when being accessed without authorisation. Of course an exothermic reaction of any kind, i.e. fire -- although effective -- is too dangerous and illegal for posting. However, some material decompose and change when exposed to water rapidly. One mechanism is to use washable ink which immediately runs when exposed to water. The envelope will detect tampering and releases an ink destructive agent using a rapid distribution system such as a slightly pressured water pouch.
To detect the penetration of the envelop the envelop material has woven or printed strands of an electrical conductor in horizontal and vertical strips seperated by a thin insulator. When either of these layer is torn the electrical properties ( resistance, capacity or inductivity ) change and can be detected. A simple printed-on number pad can be used to disarm the mechanism upon receiving of the item. Then the envelope can be unsealed without impacting the ink. Unfortuntely, the ink will destroy over time hence the document needs to be treated on the receiving side to permanently fix.
[1] United States Patent 3895755, http://www.wikipatents.com/3895755.html
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