Th Dutch Postal Service (PostNL) has introduced a kind of post stamp: digital post stamps. In the Netherlands, people are sending fewer and fewer letters and so fewer people have post stamp in-house. So if you has to send a letter, you need to go a shop to buy an entire sheet of post stamps (the last time I have bought post stamps, has been five years ago or so). It’s not possible to buy single stamps.
However, PostNL has come with a solution. People can now download an app on their cell phone. With this app they can buy a code, instead of putting a stamp you write that code on your letter’s envelope. The code is scant at the post center and recognised as stamp. The code is made up of nine numbers and letters (which gives more than 10 billion combinations, so the chance to guess a valid code are nill).
For those who are able to read Dutch: http://nos.nl/artikel/509787-postzegel-niet-meer-nodig-met-app.html (If you cannot read Dutch, you can use Google Translate; although this wouldn’t give you a fully correct translation, however it’s mostly understandable).