Verdi said delays to deliveries could not be ruled out as a result of the strikes.
Amazon itself said customers could order up until midnight on Dec. 21 to get gifts in time for Christmas, or even on Dec.23 or 24 if they pay for express delivery.
"We deliver reliably,'' an Amazon spokeswoman said, adding that only a small minority of workers had joined the strikes, with around 19,000 employees working normally.
Last year, Amazon orders in Germany peaked on Dec. 15, when customers bought 4.6 million items, or 53 per second.
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Verdi has organised frequent strikes at Amazon since May 2013 as it seeks to force Amazon to raise pay for workers at its distribution centers in accordance with collective bargaining agreements across Germany's mail order and retail industry.
Amazon has repeatedly rejected the union's demands, saying it regards warehouse staff as logistics workers and says they receive above-average pay by the standards of that industry.
Amazon has previously said the long-running dispute has not affected deliveries as the vast majority of workers in Germany have not joined the strikes and it can draw on a European network of 28 warehouses in seven countries.
The U.S. company employs almost 10,000 warehouse staff in Germany, its second-biggest market behind the United States, as well as more than 10,000 seasonal workers.
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