Question about Unusual Package Received

Question: This evening I received a package from AT&T. It was a device that I returned in November of 2025 for a trade-in for a Next Up Anytime deal accompanied by a letter that said it wasn't accepted because it was received after the deadline. When I go into my account it shows the device was returned, received, and accepted by AT&T. Being that it is August of 2026, I was not at all expecting this package especially of a phone that I returned in November of 2025. Letter also stated that I would be charged for the remaining balance of the phone. Thoughts? Ideas on next steps.