I am travelling from Hangzhou to Shanghai by fast train.

And then want to get the airport shuttle bus 1 from Hongqiao East Transportation Hub to Pudong International Airport.
It will be Friday 30th September just before the public holiday. I need to be at the airport by 21:00 (3 hours before the flight). How do I buy the bus ticket? Do I have to buy it in advance or can I buy it just before getting on the bus? If I leave plenty of time ( 1-2 extra hours) will it be a problem to get a bus to the airport due to the number of people travelling on this day?