Reserving bus tickets (Shinjuku - 5th station, return) online for international travellers

by Alena
(Australia)

Hi Gary,
My partner and I are climbing Fuji San, Hut booked on the Yoshida trail on the 24th July 2018. I read your link regarding the bus from Shinjuku to 5th station, (very helpful, thank you).
I see you can't book any earlier than 1 month prior to travel.
Will we have any issues reserving tickets being from abroad (Australia)?
Also, all going well, do we need to reserve a time for return bus trip,
just thinking we may not know an exact time we return to the 5th station from our descent.
thank you in advance,
Alena