The first £600 energy payments are being rolled out in Northern Ireland.
The money includes a £400 payment as part of a UK-wide support scheme and an additional £200 in recognition of Northern Ireland’s dependence on home heating oil.
Electricity companies are being funded directly for the scheme to deliver money to households amid the cost-of-living crisis.
Those who pay their bills by direct debit will receive the £600 in their bank accounts while other customers will be sent a voucher.
The mailout of the vouchers is taking place in tranches over the next four weeks and will be completed by the end of February.
The vouchers can only be redeemed at post offices.
The Post Office said it has worked with the electricity suppliers to prioritise the delivery of vouchers to households on the customer care register.
Each gas and electricity supplier is required to compile and maintain a register which stores details of consumers who merit special treatment on account of age, disability or chronic illness.
As the vouchers start to arrive on Monday, the Post Office has urged everyone due to receive a voucher to pay close attention to their post and to be careful not to accidentally throw their voucher away.
With an estimated 75% of households in Northern Ireland not paying energy bills by direct debit, it is expected that post offices will be busier than usual during the rollout.
Consumers in the rest of the UK began to receive energy support payments last year, with the political impasse at Stormont blamed for the delay in the rollout in Northern Ireland.
Andrew Goddard, head of payments for the Post Office, described the payments as a “major logistical exercise”.
“We are working very hard to distribute the vouchers as quickly as possible,” he said.
“We are working with the energy companies to distribute all vouchers by the end of February so please be patient and keep an eye out for yours to arrive.
“The only way to redeem the voucher is at a post office. The good news is that when you do receive it, our branches are open long hours and postmasters and their teams will be working flat out to process them and support their communities.
“Please do read the voucher very carefully and ensure you take it, plus the forms of ID specified, to ensure the teams can process your voucher and you receive your money. There are common questions and answers included with the voucher as well as information on what to do if you need to claim the voucher on behalf of someone else.
“We also encourage customers to receive your money and deposit this straight into your bank account. We have an agreement with all the major banks and staff at the branch will be able to do this for you there and then.
“Your energy provider may contact you to say your voucher should be arriving in the post soon. But the actual voucher will only ever be sent by post. The voucher will never be sent by email, text or other route.”
Mark Gibson, external affairs manager at Post Office NI, said they are concerned around personal security for people receiving their money.
He emphasised that the only way the vouchers can be redeemed is at the Post Office.
“If anybody contacts you purporting to be from Post Office, be it be text, telephone, email, it is not from us, it is a scam so just ignore it,” he said.
“We will not ask you to enter any details or anything like that – the only way you can redeem this (voucher) is through the Post Office.”
Further information can be found at postoffice.co.uk/costofliving