Hi there,
If you are booking a seat online using the JR-East reservation system you must enter a credit card number to place the reservation. Once you place your online reservation you must pick up the reservation ticket 1 day before your booked travel at the latest.
- If you do not pick up your ticket 1 day in advance, JR-East will charge the reservation fee to your credit card. The total amount charged depends on the route and seat class, but can be around ¥1000 for the Tokyo to Kyoto shinkansen in standard class.
- If you do pick up your ticket on time then JR-East will not charge you a reservation fee.
My opinion of the online JR-East reservations
The possibility of a charge unless you pick up your reservation ticket 1 day in advance is the main reason I do not recommend people to use the JR-East online reservation system unless they are very sure they can pick up the reserved ticket in advance.
I recommend that you place all your reservations for free at the train stations once you get to Japan and have your Japan Rail Pass. No credit card is required, and you can get all your tickets at once. Unless you are extremely concerned about getting a specific train there are generally many seats available for reservation.
As the reservations once you are in Japan with your Japan Rail Pass are free, you are able to change them any time by simply visiting the nearest JR station ticket office.
We have a good blog post about making seat reservations once you are in Japan here.
Hope this helps!