Individual measurement can never be correct.

On the other hand, the supposed transparency regarding stated shipping costs is also only apparent...

Well, the trend is that consumers want free delivery. But it costs someone...

How is the customer/layperson supposed to know that a measurement takes significantly more effort than over-measuring!? I also don’t believe that the OP asked/required the craftsman to take a measurement!? In my opinion, it is part of customer- and service-oriented behavior to either inform the customer about the situation in advance or to clearly present it afterwards in the offer.

I see it a bit differently with a plasterer. He mainly works for companies like general contractors and such. And ultimately, he did provide good customer service: he explained it and referred to the DIN standard. I’m just imagining that I presume to tune my PC, go to the specialist (PC store) and tell him “I want memory, have this device, and so on.” Then he gives me a part (which would also be the right one) and wants to sell it to me. And I look at him in surprise because he doesn’t explain the installation. It might be an incongruent example, but it fits with my statement that an end consumer in the big wide world of the specialist dealer or craftsman doesn’t immediately get what he expects. The end consumer omits the important service provider, his contact person who speaks his language and thus cares, because he wants to save. That there are limits or few intersections in communication, that is what he has to expect.

I, in turn, cannot understand how someone cannot understand that as a customer you want a reasonably sensible cost estimate.

Well, that's exactly what I'm talking about: a sensible cost estimate is one in which you do not have to include a calculation effort that, WITHOUT GAIN IN ACCURACY, would be more inefficient. It's not carelessness, but an EXPERIENCE VALUE where a SUITABLE, appropriate simplification of determination is applied. No one intends to violate any need for accuracy by disregard. It is not an experience value of up to what size of openings customers tolerate over-measuring - but where the optimal threshold lies in order to make RESULT-HARMLESS simplifications.

We will have to have our ground floor replastered at some point in the next 2-3 years. [...] I’d best measure it myself beforehand and then ask 2-3 companies with these figures. Otherwise everyone calculates their own little story and I can't compare it.

You’re welcome to do that. The vocational school teacher of your plasterers will be pleased to demonstrate, using the concrete example of your reference facades, how little the calculation (in the RESULT !) was "distorted" when henceforth openings up to 2.4 or 2.6 square meters (or quite revolutionarily: not at all) are over-measured. Clothing ready-to-wear sizes are also reviewed every few decades. So this heresy is not wrong. Why-how-what-whodidn'task-staystupid, as it is already said on Sesame Street. I only wish to defend the standard-compliant oversize measures against any suspicion of cunning (technical clerk, related to no craftsmen except butchers).

So measuring over is usually done for such a (long) time that it is absolutely obvious to every professional that he does not explain it separately.

In our house builder’s case, however, the contract specifically pointed out that openings larger than 2.5m² are also measured over (this applied to the 10cm thick solid wood walls). Here, the explanation why was also given (planning, sawing, corners, edges, reveals, etc.)
We took it thoughtfully and accepted it :p

That’s why I NEVER sell individual parts to end customers :rolleyes:
They actually often expect that (hours of advice regarding installation etc.) for free in the IT field. But I would like to have €8 net per unit. That conflicts and then no longer makes fun for me

Whoever takes on tenders and construction management themselves should be roughly familiar with how calculations are made. A cost estimate may be exceeded or undershot. This can well be 20%. Basic knowledge should also be present about how work is carried out and with which materials. "Let's make the wall purple" is probably something very vague. If you do not like the manner of a craftsman, you should look for another one. If you do not get along and do not trust each other, that is a bad foundation.

That's why I NEVER sell individual parts to end customers :rolleyes: They actually often expect that (hours of consultation regarding installation, etc.) to be free in the IT field. But I would like to have €8 net per unit. That conflicts and then it's no longer fun for me

I have full understanding. I also once ran a freelance service. In photographic technology there is copyright. You sell usage rights. That is stated in the offer. After the invoice, everything went well, he was surprised by my note forbidding photo distribution after blabla copyright etc., so a 5-liner. I explained the situation to him. He didn't want to understand. Didn't want to. Should I now explain to him all the copyright laws? The whole composition, reasons, etc.? The topic is quite complex. He had the information he needed. I then said he should adhere to it and if not, then he would be committing an offense. Was I unfriendly to the customer there? I think at a certain point in time and effort the customer has to contribute something themselves, to understand or at least accept it. I don't mean the OP, who simply asked here. All good. But the tenor that the payer demands all rights down to the smallest fine print, I don't agree with. He himself is obliged to understand the contract he wants to enter into.

Correct, one of our legal principles is: Ignorance is no excuse for punishment.