They can and will, just happened to us. When I rented a car our flight times were not set so I rented a car for the time frame. Well the flights I got had us picking up a car before noon but returning after 4. The price for the extra hours was actually more the keeping the original rental and with Florida August thumder storms out of Tampa I decided to keep the original rental in case of delays. When I returned the car I was charged an additional $80, which I flipped over plus wanted to charge for gas after we filled up, and yes had a slip. I did get the gas charged removed, but the counter girl would not remove the charge of the rental, reason was because we returned early. After we returned I e-mailed my complaint (never received a reply), and also called but they would not budge. So I would be careful of renting for an extra day, I will not, also will never use this rental agency. Not because of the charges but because of the poor customer responce I received. Also the girl at the counter was very rude.