They snuck in $30 a day upgrade into the contract. I prepaid for a full sized rental on priceline and picked up my rental car using one of those video kiosks. The agent on the kiosk gave me a car, and told me to approve the charges on the kiosk screen. What he NEVER told me was that he upgraded me to premium, or that I would be charged extra $30 a day for it.
I called their customer service, and they told me that it's my fault for not reading the contract. So if you're going to rent from Hertz, always read the contract carefully to make sure they didn't sneak anything in, even if you've already prepaid for everything. NEVER assume that you're getting only what you asked for. You could be in for a nasty surprise.