Kurobe OPtion Ticket vs Ordinary Ticket Reservation via Web

HI,

I understand that Kurobe Option ticket (for Alpine trip) is offered in JR Station, which is cheaper than the ordinary ticket offered via website. I'll try to ask the Kaigai/Tateyama customer desk, they say that I don't have to worry about Kurobe Option ticket will be sold out as I will buy it on 12 May 2016, just 3 days before my plan to do Alpine trip on 15 May 2016. However, I also keep checking the availability of seats via ordinary website: https://tateyama.kurobe.alpen-route.com/ticket/en.html, so I concern that 15 May 2016 falls on Sunday and when I check the time the availability for 8:40 and 9:00 has been sold out. Although my choice of time is on 7:00 and 7:20, but I am afraid that it will be qucikly sold out as well. So my question: is it better for me to book asap via ordinary website (not enjoying discount offered by Kurobe Option ticket) or can I still wait to buy Kurobe Option ticket on the day of my arrival in Japan on 12 May 2016, because the company has reserve some seats for Kurobe Option Ticket holders, beyond what is offered via ordinary website.