看你是那家公司的tracking number。 Priority mail tracking number

means nothing.  You can ask your buyer to contact the local postal office to locate the package.  If USPS lost the package, you are out of luck and USPS will not compensate you for the lost of the package unless you bought the insurance at the time of purchase of the service. If it is sent by ExpressMail, there is $100 insurance by default. Similiar if your package is sent by FedEx and UPS, there is $100 insurance.

Hope this info helps.