Bookings are always billed to the default credit card on the account.
A hold will initially be placed on the default credit card for the cost of the trip. After a short period, the charge will be directly applied to your credit card.
If there are any promotions, discounts, and driving credit available on the account, it will automatically be applied at the time of booking.
If backup payments are enabled on your account and your default card declines the charge, the next payment method on your account will be charged for any outstanding amounts.
Additional mileage costs and traffic violations will be added to the trip charges once they are received and processed by Zipcar. It is common for these charges to appear several weeks after the trip.