Hello TobiRoxy
Such forms always come from BG Bau. First you have to fill them out, then a big invoice follows. And they are not stingy.
You can take out insurance with them for yourself at exorbitant prices, but you don’t have to. For helpers you have to (if you declare them) take out insurance at the same prices. If you didn’t have any, that’s settled. The estimate is just a rough guess plus a good surcharge. Maybe you disposed of the letter without reading it carefully, or it simply got lost.
I also lean towards the tip to call, explain that only you work there occasionally, otherwise only commercially, and suggest that they send the letter to you again. The current invoice should be canceled and wait until the new form is filled out by you (of course very promptly) and received by them.
Steven