Stamps go digital – With the innovative crypto stamp, Austrian Post is for the first time offering a stamp which also exists as a digital asset (crypto collectible) in the Ethereum blockchain, and is thus breaking completely new ground. The blockchain is a special form of decentralised data storage. For this, data are not stored in a single computer centre but are distributed across many independent computers. A sophisticated mechanism ensures that completed transactions cannot be subsequently manipulated. Using the QR code printed on the stamp, the owner can look at the digital version of his/her crypto stamp in the Ethereum blockchain and then transfer it to other users.
More info here: https://wiki.coin.community/Austrian_Post_-_Crypto_Stamp