In 2013 PostNL introduced the “Postzegelcode” (Stamp Code). Simply put: by writing a number combination on the envelope after buying this code through the PostNL app, the letter could be put in the mail without stamps because the postage was correctly paid for. Initially, PostNL asked an extra 21 cents(!) for this service.
That of course did not work and the recent state of affairs was that people receive a discount when using a stamp code. PostNL was actively advertising this and the discount stood at 4 cents.