I think we are being a bit harsh on OP.

1. I agree that, in the absence of any express statement to the contrary, California law doesn't apply.

2. The fees were €40 (service charge - that could be a translation/restatement of the location fee and so I would argue that it is valid) but also a one-way fee (not included in the AutoEurope quotation) and €17.85 VAT.

The issue here is that the contract is with AutoEurope but the charges were levied by Hertz. The question therefore is whether the charges are valid (IMO they are if OP agreed to them) and what the remedy is against AutoEurope.

The correct remedy would have been to address the issue with AutoEurope when first made aware. AutoEurope is in breach - you can't quote for a one-way rental, omit the drop fee and then add it on without referring to it as the drop fee is what makes the rental one-way in the first place.

Similarly, if no taxation is either stated to be excluded or included, it must be included as the rental can't be supplied without it.