It would only cost a few dollars but it requires custom programming work that has not been done yet. It would be very effective. Battery powered RFID can transmit pretty far. The device can trigger with a photovoltaic cell (solar panel), since when the package is opened it is exposed to light. It should trigger the wipe of volatile memory that holds the seed for a PRNG. The current state of the PRNG should transmit once every minute or something, after a certain time delay has passed (so it only starts to transmit after it is in the box, not on its route to the box). You can then just use an RFID wand near the box (100 meter radius?) to see the broadcast, and compare the PRNG state to what it should be that number of minutes after the device was set. Keep taking measurements and the more that it matches up the more certain you can be that it wasn't intercepted. If customs opens the package the seed is wiped from the memory so customs can not reverse engineer the device or determine what the state of the PRNG should be. If the device doesn't transmit or the transmission doesn't match up to what it should be, you can assume the photovoltaic cell was triggered and thus the pack was opened. Enelysion is the one who introduced this idea, although I am pretty sure he already knew gem smugglers who were using it. My idea was for moving bulk drugs nationally via third party moving companies, with hidden drugs in some of the things being moved. Also with hidden cameras that transmit via cellular network, so you can remotely monitor the shipment for interceptions in that way. Even if there isn't constant coverage on the moving trucks route, you could have it store video and transmit when there is coverage. You don't even need to transmit everything, you can have it so it only transmits when the video changes (like if there is light entering into the truck after it is opened..if there is no change detected it doesn't need to transmit the same image over and over). Enelysions technique is more practical though, and works for international orders.