For those of you who doesn't know, you can return your octopus cards in the train information booth at the airport and get your 50$ deposit back. This however needs to be done before passport control as there is no similar booth inside. I didn't test this for myself though, as I was running short for my flight. If anyone knows otherwise he/she can update here. Sent from my Android based tablet. This on screen keyboard is challenging and may cause typos.