You can only book reservations in selected car parks. You will be asked to select the entry and exit time and pay in full for your parking. As payment is taken in advance you will not be charged twice via the RingGo app, unless you were to stay longer than the exit time selected.
Please note: As this is a new feature, the reservation will not appear in the RingGo app, so please do not be alarmed. You just need to ensure the vehicle registration number in the confirmation email is the same one you are parking on the day!