We rented a car for just over 3 weeks via Gemut. The rental was booked through AVIS, & the paperwork and payment was done in advance, I just had to present my voucher at the rental counter in Frankfurt. Included was a free vehicle upgrade from basic to intermediate size car. I had booked a manual transmission & the CSA asked if I wanted an automatic, not thinking much about it, I agreed. When we later got to our car it was a basic (Ford Focus), far too small for 3 adults & luggage. I returned to the counter to inquire why I was not given an intermediate vehicle, and was told by the CSA that the "automatic" was the upgrade. I pointed out that the voucher clearly stated intermediate and that a Focus does not meet that category, as to an automatic being an "upgrade" I was happy to remain with a manual transmission. She grudgingly gave us a Cuga (Ford Escape), it was still an "automatic", and we went off on our adventure.
We returned the vehicle without incident, and since it was prior to AVIS staff being on duty, (0500hrs), I took photos of the vehicle. On my next CC bill, I found that AVIS had slipped an additional 238.00 Euro charge onto my credit card for a vehicle upgrade as well as a pre-paid fill option, (neither of which were requested). I emailed Bob at Gemut about the charges and also supplied a photo of the instrument cluster showing mileage and the full tank on return, (which would negate the inference that we intended to use the pre-paid fill option). Within a day, Bob had secured a full refund from AVIS to my CC, and an apology from AVIS.
FWIW, renting via Gemut was not only more economical, but this issue was likely solved much more easily than I could have done with AVIS in Germany & my CC company directly. Bottom line, if renting in Europe, take vehicle pictures before & after, be wary of upgrades, & only rent from Gemut!