Union members at more than 100 Crown Post Offices across the UK plan to strike on July 11.
Members of the Communication Workers Union (CWU) are planning the one-day industrial action, forcing some the Crown Post Offices to close.
The union said its members had rejected a pay offer it said was worth three percent with effect from April and a £500 lump sum, which the CWU said was well below inflation.
It will be the third national strike by Post Office workers this year.
Union members took action over the Platinum Jubilee weekend.
CWU assistant secretary Andy Furey said: "No worker wants to be in this situation, but Post Office bosses can’t be surprised that callous decisions are challenged by our members.
"This dispute is about dignity and respect for hard-working employees - essential public servants who, as key workers, provided unprecedented customer service during the pandemic.
"There is more than enough money for a reasonable pay rise - implementing this pay cut is a management choice, not a necessity."
A Post Office spokeperson said that the vast majority of its 11,500 branches will be unaffected by the CWU decision to strike on July 11 and will be open throughout the day.
"There are 114 branches, typically in city centres, that are directly managed byPost Office and on previous strike days over a third have opened as usual.
"We’re disappointed that the CWU have made the decision to strike but remain hopeful that we can reach a pay agreement soon."