For reservations that are less than 8 hours long, you can cancel or shorten your reservation up to 3 hours before the start time without charge. For reservations that are 8 hours or longer, you have to do it at least 24 hours in advance.
If you don't cancel or shorten with the required advance notice, you will be responsible for the full amount of the reservation.
Note that there's a 30-minute window after you book your car where you can make any changes without incurring a charge.
With Zipcar Flex, there is no cancellation fee. Once the 15 minute hold has ended if you haven't started your trip then the car will be released. You can cancel your hold at any time within the 15 minutes. If you start your trip then you will be charged for the amount of time you have used your Zipcar until you press End Trip.