German mail system got rid of the need for stamps a while ago. Using this ever since. You can buy a stamp via app and the stamp is just a code you write with pen on the letter. Link unfortunately in German: https://www.paketda.de/frankieren/mobile-briefmarke.html
Swiss post has a similar thing, you can buy a stamp (on the form of a 9-letter code) via SMS. Never used but seems it works. No barcodes for regular stamps for now, but datamatrix barcodes are used since more than 15 years for bulk mail and print-at-home stamps.