Solution to: Sending Secrets
Notary Big puts the document in the metal tube, locks the lid with one of his padlocks and sends it to notary Small. Notary Small adds one of his padlocks to the lid and sends the tube back to notary Big. Notary Big removes his padlock from the lid and sends the tube back again to notary Small. Notary Small can then open the tube by removing his own padlock again.