Question for Green Bus

Is it possible to make a reservation online for 2 persons for the VIP bus from Chiang Mai to Chiang Rai? I can pay by credit card now, or in cash at the bus station. I will arrive June 10th in the morning, on a Green Bus from Nakhonpathom, and need to continue to Chiang Rai. I am trying to make a reservation from the USA, because I am afraid if I wait until I arrive in Chiang Mai, the bus may be sold out. Thank you.