Members of the Communication Workers Union (CWU) at Crown Post Offices – the larger branches often sited on high streets – will walk out on Saturday and in administration and supply chain networks on Monday.
The strikes follow industrial action earlier this month.
The union said Post Office management is insisting on a pay freeze for 2021-22 and a “marginally improved” pay offer for 2022.
CWU assistant secretary Andy Furey said: “Our members do not want to be in this situation, but they won’t accept humiliation either.
“Our members worked hard to deliver an excellent annual profit for the company – affordability isn’t management’s problem here.
“Frankly, their degrading excuse for an offer has only hardened members, who won’t tolerate a collapse in their living standards.
“We thank the public for their understanding and support, and we urge Post Office to get round the table and thrash out a real settlement that treats key worker heroes with the respect they deserve.”
Around 3,500 union members are involved in the dispute.
A Post Office spokesperson said: “We want to assure our customers that the vast majority of our branches are unaffected by CWU strike action on Saturday.
“There are 114 branches, typically in city centres, that are directly managed by Post Office.
“Over two thirds would normally only be open between 9am and 12.30pm on a Saturday. We apologise to any customers who are inconvenienced by a closure.”