EDIT: Support has solved all the issues, thank you!
This is the second time I rent a van with Sixt in Munich. Last time Sixt counted wrong amount of kilometers in my final invoice, but the customer support helped me solve the issue.
This time, again, the receptionist is friendly and helpful, the car is nice. But there is a mistake in the invoice again! This time I rented a van for one day (10 am 13.04 to 8 am 14.04) and returned the car on the same!! day — 13.04 late at night (around 10 pm). I've put the keys in the Schlüsselrückgabe box. Later in the invoice the return time was 14.04, 5pm. Obviously, it's out of one day window and they counted it as two days. So now I have to pay extra amount for two days for the car I returned on the same day.
SO I can only recommend to return the car keys to a real person in the office during the working hours, because otherwise they can charge you an extra day.
I will edit this review and change the stars count as soon as the issue is solved. Until then cannot recommend.