In order to pickup earlier, the vehicle needs to actually be available and not booked. Also if you must pick up before the dedicated start time of your booking, you simply pay the pro-rata cost of the additional time which will be added to your booking cost.
If you want to pick up your booked vehicle more than 15 minutes earlier than the booking start time, you’ll need to send a message in system to ensure that another member has not booked the before you. Your final booking cost will be amended to cover the additional time.