Fuji Hakone Pass + Highway Buses Reservation

Hi all, this has been puzzling me as I am unable to get much information about the usage of this pass and its reservation, especially highway buses.

I am planning to travel to Kawaguchiko from Shinjuku and end the tour at Hakone before I continue my journey back to Kanto. Since the pass will cover one way bus (Shinjuku ~ Kawaguchiko) and train trip (Odawara ~ Shinjuku) in the loop, I would like to know if I make a reservation for the one way bus ticket online, will I be paying double when I get the pass at Odakyu service center?

The online reservation seems to be cashless booking and you only pay at the service center while activating the booking there. How will they ensure that the pass is used to cover my reservation?

I also read that it is risky to end the tour at Kawaguchiko as the highway bus is always fully reserved. I don't wanna end up atKawaguchiko for another day in case I miss any of the buses back to Tokyo.
Any suggestion?

Why not make the reservation/s when you buy the pass at Shinjuku?

I have only a day to make the reservation, arrive at Tokyo on 21/11 and the planned Kawaguchiko-Hakone will be on 22/11.

I did the opposite route last time. Just book the seat online (not need to pay or give CC). When you board the bus, just show the booking and Fuji-Hakone Pass to the driver

Thanks :)