If you wish to cancel your reservation (after the purchase contact is created), there are no cancellation fees for cancellations up until midnight (24:00, Japan Standard Time) 2 days before the traveling date, but cancellation fees are applied 310 yen for 1 seat after this time.
Holders of the excursion type pass, JAPAN RAIL PASS or JR EAST PASS, are also liable to pay cancellation fees if their reservations are cancelled.