WDVS from a specialist company! Additional costs for a few cm more insulation?

It's not about whether it is economical! For me, the question is what a fair additional price for using 2 or 4 cm more insulation is.

Whether that is fair or not, no one here can answer, because we do not know the internal processes of the provider.

What matters is what additional effort arises there compared to the standard variant. Does he just pull the thicker insulation out of the drawer and offer it as standard, like you can choose between diesel and premium at the gas station? Or does he never do that, and now has to recalculate everything anew, from purchasing to transport to construction execution and possibly new training of the workers on site? We don't know. Therefore: Get two or more offers and then choose the "cheapest" (not the cheapest). If you only have or can get one offer (developer etc.), then it is what it is and it falls back on the more important (!) question of meaningfulness, which can of course be answered in terms of economy.