Where can I buy the bus ticket back to Hangzhou Airport?

Arriving at Hangzhou airport at 7.30 in morning. Flight to Guilin is not until 10 at night. I want to go through imigration and take the shuttle bus to the Merchant Marco Hotel. Visit West Lake and then go to the Zhongshan Hotel to catch the coach back to the Airport. Can I buy the return ticket on the bus at the Zhongshan Hotel