It was strange, I had read a review with regards to Budget today. I had the same experience with a pre-paid agreement through Hotwire.com. I used Hotwire to bid and pay for the car in advance (approximately $130.00). The agent turned around and recharged me the amount plus an upgrade fee of about $40.00. I was pissed off. Furthermore, they would not help me with the child's seat in the car.
I did manage to contact me credit card company to have the charges reversed and was successful. Budget did not relent. They wanted to recharge me the car rental plus an upgrade.
Never again!