2b. (free of charge) Mahnverfahren einleiten: Start dunning procedure
(you can start a Mahnverfahren online https://www.online-mahnantrag.de, or a lawyer can do it for you)
Actually, a gerichtliches Mahnverfahren is not free of charge. It has a fee. But should you be successfull, the opponent has to take that fee as well. If not, it is up to you. Costs can be found here: https://www.online-mahnantrag.de/omahn/gerichtsgebuehr.jsp
I had used it once and received all my money, including the fee, from the opponent.