Whilst you're obviously correct in terms of what the pricing actually should look like, I disagree with your last statement. If Hertz gives me a document when I rent the vehicle saying that I will be charged $X per mile, then proceeds to charge me 4 time that much when I return it, then clearly what they have charged me is not correct!

If the contract had stated I was going to be charged $35, or even if it hadn't stated anything at all, then I almost certainly would have gone to the effort to recharge the vehicle. It's the fact their own documentation stated I was only going to be charged $8 (+tax) that stopped me doing so.