Six days of strike by CWU union members are due in December due to a dispute over pay and conditions.
It has moved the final recommended deadline for sending international mail to 3 December from 10 December for places including Greece, Eastern Europe and Turkey.
Nick Landon, Royal Mail's chief commercial officer, said the CWU was striking at our busiest time, holding Christmas to ransom for our customers, businesses and families across the country. She also urged people to post parcels as soon as possible, as the post office customer experience director said that December 12 was now likely to be even busier in Post Office branches.
The union says the latest Royal Mail pay offer of 9% spread over two years lags behind the pace of price rises, which is currently more than 11%.
Changes to working conditions including start times and compulsory Sunday working are what the CWU describes as the Uberisation of the postal service.