Incumbents Lansons, Eulogy and Citigate have retained their places from the previous roster following a lengthy tender process, which attracted more than 120 applications.
Blue Rubicon and M&C Saatchi PR, the latter of which declined to repitch, are not on the new roster.
Salt, Third City, Unity, Citypress and Cohn & Wolfe have been recruited as the Post Office, which has promised no more closure programmes for its 11,500 branches, adjusts to its recent separation from privatised Royal Mail.
Nina Arnott, head of PR at the Post Office, said: "We are delighted to be working with such ambitious and creative agencies at an exciting time for the Post Office, as we transform our branches and develop the multi-channel strategy that will secure the future of our business.
"With over 11,500 branches throughout the UK and over 170 products and services, the Post Office turns over £1bn a year and has a bigger network than all the banks and building societies combined. Around 18 million people visit us each week. We are an important UK organisation and a trusted high street brand."
The roster is divided into corporate and crisis work, brand awareness and the different product ranges offered by the company.
Citigate, Salt and Third City will handle corporate and crisis work, which has an estimated budget of between £100,000 and £1 million.
Lansons, Cohn & Wolfe and Citypress will cover financial services, travel products and brand awareness, which has an estimated budget of between £500,000 and £1 million.
Citigate, Eulogy and Unity will cover telephony, mail, retail, brand awareness and seasonal campaign, which has an estimated budget of between £100,000 and £1 million.
Arnott, who stated that budgets would be at the "lower end" of these ranges, added: "We’ve chosen agencies which will provide us with a wide range of recent big brand experience within our markets and which also understand the unique social role the Post Office plays in the heart of our communities."
The Post Office's revenue for the first half of its financial year dropped by £21 million to £604 million. Speaking in December following the announcement of this result, CEO Paula Vennells pointed to a "period of great change and innovation" for Post Office but said she was "cautiously optimistic" that the economic climate was improving.