Trade-In Problem

Wondering if anyone has had this issue...

I took advantage of the recent trade-in offers and decided to upgrade from my iPhone 11 Pro. Apple sent me a trade-in kit and I shipped the device back. I just received an email stating that the device was not found in the box. I know it was in the box when I shipped it, so either it was taken during shipping or the person who opened it took it. I would think they would inspect the box on arrival to ensure it wasn't tampered with, but who knows I guess. I'm wondering what I can do. I did look up the tracking number for the trade-in kit to my house and the shipping of the device back to Apple and the weight difference is right at the weight of the phone, but I don't know if the weight listed is the actual weight from the UPS store or the weight input when they created the label.

Regardless, I'm not sure how I prove the phone was in there, but it was, so unsure what to do from here.