Dear Jeab,
I have called the Sixt Reservation Hotline.
The told me I have to contact the accounting department about this issue.
The told me also that the first Reservation was a prepaid booking and normally this booking cannot be cancelled.
About the second Reservation they told me that the normal rate for this Reservation is 150,98 EUR the extra of 25 EUR could be the Standard fee for cancellations (in sum 174,98 EUR). You have to pay the invoice about your second Reservation when you drop of the car in Freilassing. So I will contact the accounting department that the decut the amount of the second invoice by the amount of the first Reservation.
As soon as I get an answer form the accounting department I will inform you.
Best wishes
Sandra and Sven