"How fentanyl flowed through the U.S. Postal Service and across the southern border ...Trump weighed in on the issue Friday, tweeting that he was “ordering all carriers,” including the Postal Service, “to SEARCH FOR & REFUSE all deliveries of Fentanyl from China (or anywhere else!).”

https://www.washingtonpost.com/investigations/2019/08/23/fentanyl-flowed-through-us-postal-service-vehicles-crossing-southern-border/