Hello,
funny, we had the same problem in 2013. Rain shower & double washbasin. In the end, we agreed in advance. Just tell the developer / seller that it is a bit too expensive, they should calculate more sharply. The question is legitimate and the contract hasn't been signed anyway.
At that time, we also tried to remove these things at first. You can forget it because, as often described here, you only get a cheap handyman, have no warranty here, and only trouble in case of emergency. The possible credits for the materials not used were a joke.
I can also partly understand the surcharges. In our case, these were charged directly by the craftsman. The more the craftsmen are pressed, the higher the surcharge. You are practically without alternatives.