Shinkansen Reservations

Hello,

I currently live in Japan so I know that I cannot get a JR pass but my family is visiting me and they can. We are going to Osaka during Golden week so I would like to reserve the seats one month before to guarantee seats. I am trying to decide which pass to get. When I reserve a shinkansen seat do I have to pay the full fee up front? If so, then it is not worth it for me to get the more expensive pass. If I can pay when they get here, so that they can show their pass I will get the more expensive one.

Thanks!