My (2 people) flight lands in NRT on a Saturday afternoon at about 2:30 pm. I'm staying at the Hyatt Regency in Shinjuku. I looked over some options, and N'EX seems like the best for us. However, when I went to the website, it looks like tickets have to be purchased at the station and can't be pre-purchased online in the US. But on the website:

https://www.jreast.co.jp/e/pass/nex_oneway.html

It shows a special for 1500 yen for one-way ticket "reserved seating". To me, reserved seating implies that I can pre-purchase the tickets, but I can't figure out how.