The company said it wanted to explain its position to the CWU, and was not making any concessions.
Last week Royal Mail chief Allan Leighton insisted he could not afford to improve the 2.5% pay offer, despite a vote for strike action from the staff.
The CWU has yet to set dates for a series of planned national walkouts.
“Royal Mail is contacting Acas, not to make any concessions in our position, but to try to explain to the union once again the very serious challenges the business now faces in an open, competitive market,” a spokesman said.
If a nationwide postal strike does go ahead, it would be the first since 1996.