In Dutch: https://www.postnl.nl/versturen/postzegels/postzegels-kopen/...

You buy a code that you write down in the location where normally the stamp would be.

Just for fun, seems easily susceptible to a man-in-the-middle attack. Just take a postcard, copy their code, discard the original card, or just send the original later than the copy. :)

Yes, but then you are stealing mail. Which is bad. For a relatively small payoff.

stealing mail is also punishable with jail time basically everywhere. they take that stuff seriously

I've never used one so someone else can explain better, but my understanding is that you get an envelope and you can use your phone to pre-pay postage online (or maybe set up your card to be charged based on what's required?) and you'll get a 9 digit code back. On the corner of all the envelopes is a 3x3 grid where you write the code and that is read by the postal system as the stamp.