Summarize this article:

Zipcar reservations can be made either online at Zipcar.com or by using the iPhone or Android app. You can also call Zipcar directly and make a reservation through a booking agent.  Reservations made by phone will be charged an additional $3.50 agent fee. The Zipcar number is 1-866-4ZIPCAR. Zipcars can be reserved for as little as 30 minutes or as long as seven days. Zipcars can only be booked for one-way trips in Boston, LA and Denver. You cannot book a one-way trip internationally. When booking a Zipcar, you are guaranteed 180 miles per trip. If you drive more than 180 miles in a 24 hour period, you will be charged an additional $0.45 for each mile.  If you’re using a premium vehicle, you will be charged $0.55 per extra mile. You can estimate your mileage for a reservation by going to your confirmation page and clicking the “details” link. If you live in Austin or San Diego, you can pay your mileage as you go. If you plan to use your Zipcar to drive into a different country, call Zipcar first to make sure that country is covered by Zipcar's insurance policy. If you no longer need a Zipcar, you can cancel or shorten your reservation, provided you do so up to 3 hours before your reservation is due to start.  For reservations that are longer than 8 hours, you must cancel at least 24 hours in advance. If you don’t cancel or shorten your reservation, you will be charged the full amount.

Summary:
Book your reservation. Estimate your mileage. Cancel your reservation.