Bus from oishida to ginzan onsen

Dear all,
I will go to Ginzan onsen from Oishida station on December26.
I want to know if I could purchase return bus ticket for going to Ginzan onsen from Oishida station so that I dont have to rush or worry not to get bus ticket when I will go back to Oishida station and what is the best time to leave Ginzan onsen as I need to catch shinkansen to Yamagata spend the night there. Tks

Bus timetable (I think the content is the same)
http://www.hanagasa-bus-taisei.co.jp/base.html
http://www.city.obanazawa.yamagata.jp/files/original/20220715225802350...
Since you pay when you get on the bus, it seems difficult to buy the return ticket.
By the way, IC cards cannot be used.
It seems that it is better to prepare coins as much as possible.
http://www.hanagasa-bus-taisei.co.jp/product1.html

Thanks for your informarion