Pls clear the immigration and custom and buy from the shop at HK airport. It is cheaper than you buy from the duty free shop in the transit area. Cant remember the shop name but it is an electronic chain store which is Apple authorized dealer. Or you can place order with them by phone and tell them your arrival time, once you clear the custom/immigration, they can wait for you at the gate to sign the credit card. Hope this help..
DG Lifestyle store Shop 5P022, Level 5, Terminal 2, Hong Kong International Airport, Lautau, Hong Kong. Business hours: (Mon - Sun) 9:00am - 9:00pm +852-2769-1288