Bus at Mt. Fuji Subaru Line 5th Station

Hi,
I'm planning to go Mt. Fuji Subaru Line 5th Station by Bus without buying the return trip ticket.
Do you know where I can buy bus ticket( from Mt. Fuji Subaru Line 5th Station to Kawaguchiko station) at Mt. Fuji Subaru Line 5th Station? Possible to buy on the actual day itself? Many thanks.

Fujisan Hiking Bus is a Rosen Bus, a similar bus running in a city. You get on a bus through the middle door (#3). At that time, you need to touch your PASMO card or pick up a Seiriken, a card with a number printed on a paper with a size of about 2cm x 5cm (#4). The terminal is Fujisan station. You or somebody should push the get-off button just after the announce notices the next stop of Kawaguchiko station (#7). On arrival to Kawaguchiko station, you pay fare by PASMO or money with the Seiriken to the box neighboring to the driver (#8).
http://bus.fujikyu.co.jp/rosen/get-on-bus

There is a roundtrip ticket for the Fujisan Hiking Bus. They sell this ticket on board when the ticketing counter is closed.
http://bus.fujikyu.co.jp/rosen/detail/id/14