After reading it carefully again, here I have the question
Does that say the home building company or the "manufacturer"?
not seen and your answer
The home building company, prefabricated house
also overlooked. I have to fairly conclude that the surcharge is excessive.
An additional cost would be justified if, for example, the surface was oiled by hand 2-3 times and additionally treated with a finishing oil, which would make the surface even more resistant.
In your case, it really seems like only the home building company wants to earn.