A further 150 energy support vouchers sent to Airtricity Energy customers are also not valid for the same reasons.
The Post Office confirmed the affected vouchers are dated January 17.
"The issue was highlighted and following an investigation, new vouchers will be re-sent,” a spokesperson said.
"We understand how important this support is for people and want to apologise to anyone who could not redeem their voucher.”
The roll-out of energy payment vouchers to customers who do not pay for their energy bills by direct debit began on Monday, January 16.
The £600 payment is intended to help households cover rising energy costs, but people can use the money as they wish.
The Community Foundation for Northern Ireland has asked those who are fortunate enough to be able to pay their bills to consider donating some, or all of, their Energy Support Payment to help others less fortunate.
The mailout of the energy vouchers is taking place over the next four weeks and is due be completed by the end of February.