Let the provider know what time you're going to be arriving at the car park when you make your reservation. This means they can make necessary arrangements so that a shuttle bus will be provided at that time and you don't have to wait. Usually there is a bus available as soon as you have parked. Sometimes the driver may want to wait a little bit for other customers who are arriving at the same time before setting off. It can also be that the bus gets full quickly due to the car park being busy, such as in high season, the provider will therefore make sure more buses are running and more staff are on hand to assist you.