no receipt after returning equipment

We received our new cable box on the 1/1/26 and were told to return old equipment but did not provide a mailing label or any instructions. We took it to the local infinity store, but they would not give us a receipt and told us one would be emailed to us the next day. After 5 days, we returned and again asked for a receipt. They said they can't provide one, but that one would be mailed within 2 weeks of returning it. Still nothing. How can I prove we returned it? And why has xfinity quite giving receipts when equipment is returned?