Belgrade-Zlatibor-Mostar-Sarajevo-Belgrade

Unfortunately, the selected Mercedes-Benz V220d had a battery warning in the middle of the trip. I contacted the rental platform's customer service, and they suggested I contact the store myself, which was disappointing. The store arranged a tow truck to take the car that night, and the next day we got a Ford CMAX. When returning the car, the store forced us to pay a penalty for not having a full tank during the car change, and informed us that they couldn't refund the rental difference, and that I would need to file a complaint with the store and the rental platform. The complaint has already been filed on the app, so let's see what happens. Where do you think the problem lies in this situation?