[holgerschurig]

Well, in most sane countries "surprise bills" are illegal.

If I bring my car for maintenance and we agree on checking brake fluid and air filters, they cannot decide "Oh, the profiles of the tires are off, we'll change them". No one would need to pay them for this, as it was not agreed upon and is therefore not part of a contract.

So your case makes no sense at all.

Except that perhaps you life in an insane country, or are not well versed in basic contract law :-)