Also I have recived this by email today:
Good Morning,
I am sincerely sorry for the problems you experienced due to branch error using the incorrect reservation.
I checked the reservation and can confirm that you did not requested the satellite navigation system and the booster seat. These were included in the reservation for a different customer, which was used for your booking. I have credited these charges today. Please allow up to 7 working days for refund of 138.03Euros to show on your credit card.
As our customer you expect a professional and efficient rental service. We pride ourselves on our high standard of vehicle rental and customer service and regret that on this occasion our usual standards were not met.
Once more I would like to apologise for the inconvenience caused to you and I would like to offer you a credit voucher for the value of 30Euros for a future rental with Sixt. Please could you confirm your postal address and I will be happy to send it as a priority.
I do hope that this experience will not deter you from renting with Sixt again in the future.
Of course "These were included in the reservation for a different customer, which was used for your booking" - doesn't sound like truth, sorry
About "a credit voucher for the value of 30Euros for a future rental with Sixt" - don't think I will ever use it, because I hope, I will not ever plan to use your strange company service.
You may just send them to my credit card if you want