Same here, just had a very annoying experience at Dollar / Terstappen in FRA.
I had made a 16 day reservation for a full size, shown as Mercedes (Dollar website) or MB or BMW (Terstappen website). Of course, I know those are only examples of available cars .... Still, at FRA Terstappen told me they don't have MB or BMW at all at the FRA station but they would be glad to offer me a Chevrolet or KIA instead. On top of that they said I needed to pay EUR 21 a day for insurance unless I could present a letter by MC addressed to me personally with reference of my cc number that MC would provide coverage. That was that and now I am in a nice Avis BMW. Their loss.