Update 3/19/20: Hong Kong Post has announced that inbound and outbound mail service has returned to normal. As of this update, the USPS has not reinstated the Priority Mail Express International Service Guarantee.
Starting Monday, February 10, 2020, the USPS has temporarily suspended the Priority Mail Express International service guarantee for letters and packages sent to China and Hong Kong. Due to widespread disruptions of air cargo and trucking delivery into China and Hong Kong caused by the coronavirus crisis, the temporary suspension will last until further notice from the USPS.
When in effect, the Priority Mail Express International service guarantee provides date-certain delivery with a money-back guarantee if your package is not delivered on time.
China Post has communicated to the USPS that any parcels or letters that have delayed transit will be stored until delivery can be safely resumed.
Stamps.com apologizes for any inconvenience caused.