Kanto Pass and Reserved Seats

Hi,

I am planning to get the Kanto pass on 12/22 but not start using it till 12/24.

Do you recommend reserving seats now on https://www.eki-net.com/ English sites now or you think it is better to make it in person?

My biggest concern is I will be going to Kusatsu via Karuizawa Station on the Asama bullet train. I have to take the bus from Kusatsu to Karuizawa and what if the bus is late because of traffic (snow) and we missed the train, would be be charge with penalty? Also will we be able to get on another train on the spot?

thanks