Each USPS Certified Mail letter is assigned a unique Certified Mail tracking number also known as a 'PIC' by the Post Office. The PIC represents the transaction date and time each Certified Mail letter was accepted into the United States Postal Service (USPS) mail stream, in-route letter tracking including proof of the final letter delivery. 

Because each Certified Mail letter must be signed for when it is delivered, a signature is collected by the Post Office letter carrier when the letter is delivered. The signature can be anyone at the delivery address or a stamp used by the company or high volume mailers like the Internal Revenue Service IRS.

If you lost your receipt, USPS has no way of knowing or finding your USPS Certified Mail letter tracking number. Each year more than 300 Million certified mail letters are sent annually.  Keeping your receipt is very important and another reason to send your Certified Mail letter online so you will have an electronic record and receipt. Web sites like Send Certified Mail, Certified Mail Labels online or Certified Mail Envelopes keep copies of your transactions and store them for 7 to 10 years.