you should have filled a Customs form?

Use that number to track - starting with "CW" I think and then several digits with "US" at the end.

I use Priority Mail International Flat Rate Box, and the Customs form number provided the tracking for us - In Mid April, I was able to see that the package got to China with an attempted delivery on March 30.  My father then used the number in China to find the package a few days ago.  When I checked the Customs number again on USPS.COM, it showed delivery date of April 17.