Initially I have my doubts as the price is slightly cheaper than usual price but this is very legit. I bought 5 tickets in total a month before.
Do print out reservation ticket to show to the person in charge. The bus platform is easy to find (platform 3), located in 1st floor at Haneda international terminal. Have to access from 2nd floor via lift/elevator. Overall it was a pleasant ride, able to see some great scenery along the way and what's important is that we have arrived safely in Kawaguchiko station. Totally recommended!