What is explanation for high car rental rates?

[SimbaDog]

I was helping a friend find a rental car for her family's trip here next week, and even with corporate discounts, etc. we found Thrifty to be the cheapest. It's all the taxes and fees on top of the daily rate that get you.... and $8.00/day for a booster seat? I'm going to go to Target and buy one for her, that I will also be able to use when I have guests - for way less than $64 for her 8 day trip!

Do you happen too know Florida's law for booster seats? How tall the child needs too be to not require one?