The strikes at Deutsche Post could delay the delivery of many millions of parcels and letters by a few days, according to the company.
On Friday, around 2.3 million parcel shipments were affected by the work stoppages nationwide, around a third of the average daily volume.
According to Swiss Post, there were around 13 million letters sent, which corresponds to around a quarter of the daily volume.
Swiss Post expects that, depending on the region, some letters and parcels will not be able to be delivered until the first half of next week.
The Verdi union had been calling for strikes in letter and parcel centers and in delivery since Thursday evening.
"A total of around 16,700 employees have so far followed the call for a strike, which means around a third of the employees currently present today," said a post spokesman on Friday.
With the work stoppages, Verdi wants to emphasize his demand for a wage increase of 15 percent.
In two rounds of negotiations there was little progress.
The Post has announced an offer for the next round of talks scheduled for February 8th and 9th.
Warning strikes are therefore unnecessary from the point of view of the post office, they are only at the expense of the customers, said the post spokesman.