The invoice contains unauthorized items, how should one proceed?

especially since with option 2 you don’t have to chase your money but the craftsman

I don’t want to know what the case is, but what unauthorized amount of money are we talking about? Is the stress with a lawyer and so on worth it?

This is how it went for me: The window maker charges X hours on the invoice (among many other things) and specifies what for. I wrote to him that according to the offer 2 of the 3 mentioned items are included with installation and asked for feedback. Waited a few days and then paid the value plausible to me for 1 of the 3 services within the discount period. Never heard anything again, so probably fine for all parties involved.