[Shinkansen reserved seats with Klook]

I've been reading about buying Shinkansen tickets from Osaka to Tokyo. I would like to get to Shinjuku. From what I understand, I need to take the Shinkansen to Shinagawa, then take a train to Shinjuku.

Are there any gotchas with using Klook? The prices seem good. Is it difficult to get a ticket from the voucher at the Shin-Osaka station?

Thanks!