Royal Mail workers will be striking for 19 days over the next two months.
The Communication Workers Union (CWU) said the strikes will cover Black Friday and Cyber Monday.
It said its "significant announcement" reflects the "level of anger" workers feel.
Royal Mail Group said more strikes would worsen its financial position and threaten workers’ job security.
When are the strikes?
The strike action has been called for 24 hours from 04:00 on October 13, 20 and 25 and November 28.
A CWU spokesman said different teams and combinations of teams will strike on other dates, which will have a knock on effect on all deliveries.
Why are Royal Mail workers striking?
The CWU general secretary, Dave Ward, said: “This is a significant announcement, but it is one which matches the level of anger our members feel at the way Royal Mail Group has treated them.
“The chief executive of Royal Mail Group is treating postal workers as if they are stupid.
“These are the same people that have kept the country connected and returned Royal Mail Group to record profit.”
What have Royal Mail said about the strikes?
A Royal Mail spokesperson said: “On September 22, Royal Mail invited CWU to enter into talks through Acas to find a resolution to our dispute on change and pay.
“This evening, rather than responding to our offer of Acas talks, the CWU announced further damaging industrial action, once again taking the path of prolonging disruption over resolution.
“Royal Mail is losing £1 million a day and must change faster in response to changing customer demands. We operate in a competitive market, and our customers have choices.
“Further strikes and resistance to transformation by CWU will only make our financial position worse, and threatens the long-term job security of our postmen and women.
“The CWU has a responsibility to recognise the reality of the situation Royal Mail faces as a business and engage urgently on the changes required.
“We apologise to our customers for the inconvenience the CWU’s continued strike action will cause. We are doing all we can to minimise any delays and keep people, businesses and the country connected.”