That's it. They have gone overboard crazy with bogus fees.
In the US, Sixt is now charging a "Counter Processing Recovery Fee" and a "Vehicle Upkeep Recovery Fee" on any rental.

And these fees are not cheap, in the $12-$15 range each, it seems (varies by location). Plus taxes, of course.
These are fees that totally go into Sixt's pocket. Not an airport fee, not a concession fee (those are charge separately, when applicable). These new fees apply also at local Sixt standalone offices not tied to any airport or hotel.
Couldn't they be a bit more honest and just increase the rental rates?
I'm not renting from Sixt in the US until this dishonest practice stops. [Or so I say. Unfortunately what is most likely to happen is that Hertz and Avis and others will admire Sixt for their brilliant move and institute similar fees.]