Not every number issued by USPS is a "tracking" number, and not all numbers are viewable online at usps.com. The only numbers viewable at usps.com, bu the public, are those for Delivery Confirmation(not a tracking service), Signature Confirmation(not a tracking service), or Express Mail(a tracking service), for Domestic Mail. Customs Labels that start with a "C" for International Mail also offer tracking to major destinations, as well as International Express Mail. If you have a Certified or Insured number, you can not track or view any delivery info online. Only your PO can see that information.