The pick-up and drop-off times you give us can only ever be rough estimates, and generally speaking as long as you arrive within half an hour of the times given then there is no need to worry. If you are going to arrive later than planned then you should call the car park operator on the number provided in your booking voucher to inform of what time you expect to arrive. If you are going to arrive significantly earlier than planned then you may also wish to inform the car park to avoid having to wait too long for your airport/port transfer.
Car park operators are used to delays and many ask for your flight/cruise details on the booking form. If on your return you find that you are delayed and are unable to inform the car park before take-off, then you should simply call as soon as you land/dock. If the shuttle bus operates on a timetable then you should make sure you have a copy of this with you. In theory the car park operator can charge you for extra time your car is in the car park, however, many car parks will in fact allow for a 'courtesy period' to allow for delays, meaning you will not be charged extra.