Individual measurement can never be correct.

Well then, it's all the more reason to get comparison offers. Just take the cheapest one then.

This is not about the price yet but about the untraceable measurement.

But it is traceable according to DIN! Only you don’t see it.
A layperson comes to the craftsman and doesn’t understand the world anymore because they are in an unfamiliar one... :rolleyes:... always the same.

Actually all said, common practice and according to DIN. You don’t have to know that as a customer on the first inquiry, but it has now been communicated to you and you know it. Now you can make further inquiries with that knowledge. 99.9% will measure and bill according to DIN ("Openings under 2.5 sqm are overmeasured" typically appears on the offer). Or you find the 0.1% who are willing to measure every opening exactly; assumption: it will cost you significantly more.

No one forces you to conclude a contract with one or the other. You now know the common practice in construction and must decide for yourself how to deal with it. Several people here have now confirmed to you that it is common practice.

The flashings and upstands are additionally calculated in running meters. So I am paying twice after all

You are not paying twice. You are paying what was offered to you. If it's too expensive for you, then get another offer.
It only gets bad for you if someone offers you 8€/m², you think it's 158, and then the bill comes to 220.
But here everything is clean and comprehensible.

Well, I think if you already make an offer based on measurements, then it should be comprehensible as well.

That's true. At the same time, one can assume that a client also deals with the matter and is willing to think along. You can't cobble together an offer outside of the standards for everyone. Besides, this also results in a loss of comparability.

What does the price have to do with that? It was a figurative comparison—filling 20 liters but paying for 40 liters certainly has nothing to do with price differences.

The so-called over-measuring serves a simplified and fair calculation. The additional effort for work preparation and execution is calculated against the area. In this way, offers become more comparable. In your tank example, it would be like wanting to pour 15 liters directly into the tank and filling 5 liters dropwise into small containers. You can list that separately or generalize it. When reinforcing walls, not every square meter requires the same effort.

The reveals and lintels are additionally calculated in running meters. So I am paying twice after all.

No, you are not paying twice, but for a different service which involves increased labor.

This is not about the price yet but about the measurement not being comprehensible.

I understand that the measurement is not comprehensible to you. Have it explained to you and do some reading. My mother also finds the operation of an iPhone not comprehensible—that doesn't mean the operation of an iPhone is generally incomprehensible.

Hampshire has hit the nail on the head and I can understand it a little better now.