Terrible Customer Service

We get up to the booth after waiting almost an hour and the girl asks how many we have in our party. I reply 2 kids and 2 adults. The girl says you are renting this car for 15 days and are going to hate it because it wont fit everything we need. She then says no problem we can give you an SUV. Which sounded great, she did all the paperwork just to find out she tried to charge me an extra 300 dollars as an upgrade fee. I told her that I don't want to spend an extra 300 when I already spent almost 500 on the vehicle already. She then replied all well enjoy the small car that wont fit everybody. I then asked for the manager and he came over and said yup nothing I can do unless you want to pay more money and then left. They said I already got "too good of a deal online that they cant do anything for me". I asked if I could cancel my rental and they told me to call online because that's who I booked through. Absolutely Terrible customer Service.