Fox Rent A Car review: Not agreed payment

We rented a car via Uber and there the booking said it will cost around 200$. When we arrived they told us that we have to pay an underaged fee as well as an insurance, that was fine for us because the Lady said the TOTAL will be around 300$ and the card will be blocked with a total of 600$ but that after the return we will get 300$ back.
After we returned the car today we got an email which said we have to pay 453,47$, what is way too much!