Like all mail, some letters or postcards bearing EUROPA stamps happen to travel underpaid and in such case the letter (or postcard) get Taxed.
On this page you see 2 such examples of France 1956 EUROPA stamps, were items were deem to be underpaid and taxed. Postcard was posted from French Army in Germany to France (and taxed), letter was posted as a First Flight but return to sender (with a tax)…