Customised French Stamps

Well, the 'stamps' on this cover are really bizarre: no face value, but only the mention: "Lettre Prioritaire 20g"... 0.o   That's what they call "customized stamps" in France.  Personally, I've still not understood how it works -_-   

From right to left:

1. Brie de Meaux, a soft dough crust cheese,

2. Marché International de Rungis, the principal market of Paris and the largest wholesale food market in the world,

3. Basilique de St. Denis, a large abbey church in Saint Denis, a northern suburb of Paris,
4. Château de Versailles, the renowned palace which was home to the monarchs of France.

Thank you Eric ...