Unfortunately, you cannot book a ticket from Shinjuku to Kawaguchiko online(https://highway-buses.jp/) if you are purchasing Fuji Hakone Pass.
Could you please purchase the ticket at a ticket counter after you arrive in Japan. To purchase Fuji Hakone Pass, please visit Odakyu Sightseeing Service Center before your departure date.
It might be better to go from Shinjuku to Hakone and Hakone to Kawaguchiko and then back to Shinjuku as it is getting very difficult to book a ticket from Shinjuku to Kawaguchiko on the spot. Please make sure to book a ticket from Kawaguchiko to Shinjuku at the ticket counter before your trip to Hakone to ecure your seat.
This is a good season to have a nice view of Mt. Fuji.
Hope you enjoy your trip in Japan.
Please share your trip on this website when you come back.
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