File a Missing Mail Search Request. Have encountered several similar occurrences in the past few months as both shipper and recipient. The packages have promptly reappeared in the system and subsequently been delivered without further delay. Another bit of advice: Don't delay! File as soon as two days have elapsed without a tracking update.
LoL... I did that on Dec 19th. Got this reply on Dec 21st. Package still not here.
This is in response to your inquiry regarding the delivery of a package. Please accept our sincere apology for any inconvenience this matter may have caused you.
The Postal Service is aware of the frustration and disappointment caused when we do not live up to our commitment of safe and reliable mail service.
Due to the high volume, your package has been delayed. I am unable to give you an updated arrival time. I want to apologize and thank you for your patience. I do not believe your package is lost, only delayed. We strive to provide a better service and have fallen short. We will continue to work towards improving. Again, I apologize.
An apology is no substitute for good service, but I want to offer one on behalf of the Postal Service. We appreciate your reporting this matter to us. It helps to know the kinds of difficulties our customers have so we can work toward improvements. The information you have provided will be shared with management as they continue their efforts to improve service performance in your area.
Thank you for the opportunity to address this matter with you. If you have any questions, please contact me.