Reader question: Shipping costs A4 envelope from Thailand to the Netherlands with airmail?

I understand that many people have never heard of it or know of its existence, but there is an 'INTERNATIONAL RESPONSE COUPON' that can be used worldwide. You can buy such a coupon at the post office and it has the value of a normalized airmail shipment. It is more expensive than a stamp in the country itself. If you think the shipment will cost more than a normalized shipment, you can simply put two in the envelope. To be valid, this coupon must be stamped on one side, left, by the post office where you purchase it. If this stamp is not there, the coupon is invalid. The same if the coupon is stamped on both the left and right, then it has been used.
We, as radio amateurs, often use this as a paid written answer to a made radio connection (QSL for those in the know). Reason: can be used just about anywhere in the world and is much less susceptible to theft than, for example, putting a 2 USD in an envelope. nor the risk of buying a stamp somewhere on the internet, which later turns out to be out of circulation.
When I used this for the first time in Thailand I thought: what kind of misery is that going to be???…. But no, even in the local post office, next to my house, they knew it. To my great surprise, I even got a few cents back because the value of the coupon was more than the rate to be paid… Never happened to me in Belgium.