The USPS will be performing scheduled maintenance on their Tracking APIs:
From: 8:00 PM ET on Sunday, January 13, 2019
To: 2:00 AM ET on Monday, January 14, 2019
As a result, during this time, you may encounter interruptions in the PB Complete Shipping APIs Tracking Subsystems (Production and Sandbox). It is also possible that the information returned to you may not be up to date. If you encounter issues, please try your operation again after the maintenance window concludes at 2:00 AM ET on January 14, 2019.
Possible Action Required:
If you experience any other issues during the maintenance window, please contact the PB Shipping API Support Team at ShippingAPISupport@pb.com