The invoice contains unauthorized items, how should one proceed?

I see this more often with architects. They properly document on the invoice/invoice copy or on a separate form which items are removed or reduced, and then new totals are calculated. This then goes to the client for approval. I assume the invoice is also sent to the craftsman again for information.

I would do it similarly. Properly document which items are removed, calculate new totals, and make the payment. At the same time as the payment, send an email to the craftsman stating that you have transferred the money but have reduced the amount by the items/positions that follow. Finally, request a credit note or a revised invoice for your records.

Definitely pay the undisputed part! For the unjustified items, send an objection by registered mail. In case of a dispute in court, it will only concern the value of the unpaid items and not the entire invoice. Such things are always credited positively for the client!

Exactly, the question of whether a claim is justified or not is too complicated for a forum.

I couldn't advise anything there at all, since the information about what "matter" it concerns is missing. There is a complete lack of neutrality.

If you stop talking to your service provider, the bickering, as you call it, will continue anyway, regardless of whether you choose option 1 or 2. There are always at least two parties involved in a quarrel. Silence is not a productive way forward; one way or another, you will have to engage in dialogue. The two options are those through which lawyers make their money and further escalation steps are initiated. I don’t find it pragmatic – not even when people have already clashed and this is the 25th point of contention. Somehow, it came to this...

I basically see it the same way as you. But we won’t manage to sort this out here. Of course, there should be communication. I just want to know what the better way is for the now short-term upcoming payment or non-payment.

It's fine, nothing needs to be rolled out here. If these two options are available, I find option 2 the lesser evil.