Hey is there any price difference between Nozomi and Hikari if I buy it at the counter?Only if you use a reserved seat. The difference is a few hundred yen. If you travel by unreserved seat, there is no price difference.
Can I buy ticket a few days before departure but without seat reservation extra charge?
Yes, it is possible to buy tickets in advance, but if you do not make seat reservations, there are not many reasons to do so.
The difference in price between reserved seats and unreserved seats is roughly 500 yen.