hello. Hope anybody could help answer these following queries. we'll be in HK for 3 days. we are 3 adults and 1 kid. we will be using MTR mostly to go to the disnyeland, ocean park and victoria peak. and on our departure, we want to ride an Airport express back to the airport. Which octupos card shawll we going to buy? how much does it cost? can we buy and have it refunded in the airpot before departur?

Thanks a lot.