Cityflyer Pre-paid Return Ticket still available?

i will go to hong kong on 21 sept 2015 and return to my country on 23 sept 2015. i will stay in Chungking Mansion area. From my research, i can take bus A21 to Nathan Road from airport. its right?
one way bus ticket 33HKD. Suddenly, i found cityflyer prepaid return ticket. From HKIA to Nathan Road return costly 55HKD ( i can save about 11HKD if i buy ticket separate). This is still available? Where to buy and how to use it? tq