How did Pinduoduo know that contacting the consumer was out of stock?

1. How did Pinduoduo contact the consumer to stock out?

Merchants inform consumers by phone, SMS, email and platform customer service tools. Explain that they can't deliver the goods, admit that they are out of stock/out of stock, ask for a replacement, or ask them to deliver the goods on other conditions (such as compensation for freight, delivery of more than X pieces/more than Y yuan, and delivery to the designated area, and consumers need to take them out), or ask them to cancel the order/refund, etc.