How much do you value your ability to rent from Hertz in the future?

You could just return it with 10% if charging before returning isn’t convenient/possible, claim that you weren’t told otherwise, that fees weren’t disclosed, and if they want to charge you any fees, dispute them with the credit card.

You could also reach out to Hertz via Twitter and ask them what the rules are for your specific rental. I had them confirm to me a while ago that all Tesla rentals have to return with 10%. That may have been superseded by the new fees others have reported, but back in the summer/fall it definite was okay and I had it in writing.