iPhone X Trade-In Stolen After Dropping It Off At USPS

I have the AT&T Next Every Year trade-in plan, and after receiving my new iPhone XS, I packed in my old iPhone X and dropped it off at USPS on 9/26/2018 at 6:09pm and got the US Mail Receipt as instructed by the trade-in instructions.

While I was waiting for the confirmation from AT&T that they received my phone, I heard from my friend that his iPhone X Trade-In phone was stolen after he shipped it out, and later received a package with a htc phone (Verizon) inside that was not his, and a letter saying he had sent the incorrect phone.

I was worried that the same thing might happen to me ever since, and then today, I received a package from Returns 13550 Independence Parkway, RLC, Fort Worth TX 76177 (999)-999-9999, and inside was a Motorola (Sprint) cell phone (not mine), and the same letter that my friend received that says "We're unable to complete your trade-in because it is the incorrect phone. We must receive the correct phone by 11/03/2018 or you will be charged for the remaining installment balance."

I need to know what happens when things like this happen.

I did everything as instructed for the trade-in.

My guess is that some of the employees at Returns is opening our trade-in packages and stealing our phones we sent, switching it with very cheap ones.

I have a picture of the return label and the tracking number on it from when I sent out my iPhone X, the US Mail Receipt, and my stolen iPhone X's UDID number.

I'd very much appreciate it if someone could look into this and give me advice.

Letter I received in the package with someone's Motorola phone