USPS First Class Package International transit times will vary depending on where you are shipping to as the local Post Office in each delivery destination can vary depending on how developed they are.
Tracking Number is a unique ID number or code assigned to package or parcel when it was shipped. Tracking Number allows you to track the progress of the Package between the source and destination location.
United States carriers: UPS, USPS, DHL, FedEx.
China Carriers: China Post, YunExpress, SF-Express