I had this happen to me a few times. First time, My husband and I went to the address it was delivered to. The people were home and had just got their mail. Opened up the package and it was a plastic toy versus a camera lens. I took photos of the outside of the envelope showing the tracking and incorrect address. Submitted it and won the case.
The other 2 times, I caught the packages before they were delivered because I am signed up with Daily Digest on the USPS website. Every morning I get a notice of what will be delivered that day. First class mail actually has a photo of the envelope. I saw that the heavy lenses I was to receive was being shipped to another address in a flat envelope. I contacted my carrier and she verified the tracking and incorrect address, she delivered them to me. I took photos of tracking and fake address and was able to win those cases.
I hope you can track that package down and solve the problem.
