Members of the Communication Workers Union (CWU) at up to 371 of the offices will walk out for 24 hours - the seventh round of industrial action since Easter.
The union said growing numbers of members of the public and shop owners were supporting its campaign to stop plans to franchise or close 74 of the Crown offices - the larger branches usually sited in high streets.
Officials have warned that jobs will be lost if the plans go ahead, adding that staff have not had a pay rise for more than two years.
Dave Ward, deputy general secretary of the CWU, said: "With the news that Post Office bosses shared £15.4 million in bonuses, their plans to close offices, cut jobs and refuse to agree a pay rise for Crown office staff is unbelievable.
"They're lavishing bonuses at the top while slashing jobs, services and pay for frontline staff. It's blatant fat-cat behaviour from management out of touch with their workforce.
"Post Office staff, customers and communities are continuing to stand up for the local Crown post office services they value so highly. The support continues to be enormous. Bosses cannot keep their heads in the sand forever.
The union said tens of thousands of people had signed petitions across the UK in opposition to Crown offices closing or moving into retail or other stores instead.
The Post Office said the Crown network is losing money and has stressed that most of the 74 offices under review will be franchised to other locations.