Airport express ticket

For a single trip, 225 HK$ is needed in all. 90 HK$ for adult and 45 for child.
Yes, you can buy the ticket from Kowloon Station or buy online.
For the back trip, if you still want to take the free shuttle, you can show them your airplane ticket to see whether you can get on the free shuttle bus.
In my opinion, if you will stay there no more than 14 days, the Airport Express Tavel Pass is suggested. After buy it, you cannot only take the express and can also take most of the railway lines for free in 3 consecutive days, except for Airport Express, Light Rail, Bus, East Rail Line First Class, Lo Wu and Lok Ma Chau). You can buy the Travel Pass with Two Airport Express Single Journeys, so that you can definitely take the shuttle bus and the transportation fee will be low.