can I only buy bullet train tickets from Shanghai to Hangzhou from Shanghai?

I'm going to arrive at Shanghai Pudong airport on May 2nd from Tianjin and planning to go directly to Hangzhou. Since they're having their labor day holiday in China and I heard people will be traveling, I want to take precautions so I will get a ticket to Hangzhou on that day. But I also want to take precautions if my flight got delayed. So any suggestions?