We rented through Discover Cars this past October. They’re a third-party car finding service (like Auto Europe), based out of Latvia. We’d never heard of them either, but on faith we paid the downpayment (likely their fee for finding the best deal in Bologna, Italy), then paid the remainder when we arrived at the actual Italian rental company 3 months later.
It’s not Discover Cars who has the vehicle, it’s the rental company they are representing. You should be able to see what that company is, although apparently it’s neither Hertz nor Sixt. And maybe the discover Cars brokered price is lower than the price of the actual company, if you went directly through it.
The rental company in Bologna had our reservation in their system, so Discover Cars met their obligation in communicating with it. The rental company then “upgraded” us for free, with a larger crossover vehicle, but we were originally looking for (and just paid for) a mini category, and then it was never going to be a convertible. My guess is that it was what they had in inventory, and that was beyond Discover Car’s control.