Kocs told the paper further talks could only happen if Deutsche Post scraps a request for Verdi members to work longer hours.
The company wants workers to work 41 hours a week instead of 38.5 hours, something Verdi says could endanger 12,500 jobs.
Voting will continue through Tuesday. Should at least 75 percent of Verdi's 130,000 members at Deutsche Post support a strike, walkouts could begin on Friday.
A series of limited strikes at mail sorting centres left tonnes of letters and bulk mail undelivered across Germany last week.