The tender is set to run over four years and is split into three lots. One covers corporate and crisis, with the budget ranging between £100,000 and £1m.
Another covers financial services, travel products and brand awareness. Worth between £500,000 and £1m, it includes consumer PR for Post Office FS products, travel insurance and FS brand awareness.
Lot three covers retail and telephony consumer PR for mails and retail, home phone and broadband, brand awareness and seasonal campaigns and is worth between £100,000 and £1m.
Tender documents seen by PRWeek state that it is looking for "agencies to support the communications department in promoting products that increase revenue, and enhance the corporate reputation to grow the business".
The Post Office split from the Royal Mail Group last year, with Royal Mail floating on the London Stock Exchange in a deal worth £3.3bn in October.
Nina Arnott, head of PR at the Post Office, said: "We would like to hear from ambitious and creative PR agencies that can help us navigate the challenges and exploit the opportunities ahead of the Post Office.
"Our ideal agencies will have lots of recent big brand experience within our markets but will also understand the unique social role the Post Office plays in the heart of our communities.
"Most importantly, we want to work with people who take pride in being the best at what they do and would be passionate about telling our story."
The current roster, which expires in May, includes Lansons Communications, Eulogy, Citigate Dewe Rogerson Limited, M&C Saatchi PR and Blue Rubicon Limited.