Need help finding a package sent via USPS Priority Mail LK169723156US

Priority package mailed 7/1/13 was sent to Canada in error by the post office and then returned to Chicago, USA to the Custom's Department to again be shipped out hopefully to China where it should have been sent in the 1st place. No one seems to know where it is. USPS says they do not provide tracking for Priority flat rate envelopes so I am out of luck. Have lost more than $100 because of the loss of this package. Have tried tracking on various China Post websites but can find no record of it after second Chicago USA, it cannot have just disappeared! Can anyone provide any help?