You must purchase an Airport Express train ticket to the airport in order to enter the check in area. You cannot just use the check in counter without purchasing a ticket. If you use an Octopus card, the fare will be deducted as you pass the turntile into the check in area. The earliest time that you can check in varies depending on which airline and destination. The system is smart enough to know not to deduct the fare the 2nd time when you return several hours later to take the train to the airport. However, there is no refund if you change your mind to use other mode of transport to the airport, e.g., taxi or bus. If you take a taxi to the Kowloon Station and the fare is HK$60 or more, you will receive a discount on the Airport Express train ticket. Ask the taxi driver for a receipt. Otherwise, there are also free shuttle buses from many hotels.