Booking a long-term rental via billiger-mietwagen.de basically works just like any other car rental booking.
The booking procedure is independent of the rental period and works according to the same pattern: You search for offers on our site and specify the location and pick-up and drop-off time. You will then see all the available offers with the included services and the total price.
If you rent the car for a long period of time, it may be the case that the rental company wants to check the car every now and then. In such cases, for rentals longer than 28 days, you will need to show the vehicle briefly at the location after 28 days.