Bus Huangshan and Hangzhou

How does bus travel 'work' in China between Hangzhou and Huangshan (Tangkou)? If we buy a roundtrip ticket to Tangkou from Hangzhou (is there enough time to buy it the night before or even day of?), do we get some kind of guarantee that we will definitely get on that bus going to and coming from? I don't want to get down to the bottom of Huangshan after our 2 day exploration only to find the bus is sold out to get back to Hangzhou (anybody got the schedule for Tangkou-Hangzhou as well as Tangkou-Tunxi-Hangzhou? I heard this route might be faster than the direct from Tangkou but it depends on when these buses leave Tangkou. We might opt for sitting for 5 hours over going 1 hr, waiting 2 hrs, then sitting for another 4hrs etc). Please and thanks!