T-Mobile lost warranty phone

My iPhone 16pro got stuck in a boot loop. So we took it to the Apple Store only to find out t-mobile cancelled that phones Apple care early. So we took it to the t-mobile store to get a new phone through a warranty claim. The store set everything up but wouldn’t take the phone saying we need to mail it in. We get the return label in the mail and promptly lose it. After talking with t-mobile multiple times they sent a new return label. We ship it, we have the receipt for dropping it off. It was delivered September 4th according to UPS tracking. They can’t find it. So after many phone calls and getting charged for the phone they lost, we have been told that there is no proof that we put the phone in the box. They have no proof there wasn’t a phone in the box. We are being told the issue is closed and we must pay $1200 for the phone. Please tell me there is something we can do. We returned the phone, it was in the box when we dropped it off, the weight of the box is on the receipt showing the correct approximate weight. I don’t know if it was stolen or they lost it after it was delivered or what. They say they don’t have cameras or any systems in place to mark when a box is returned empty and we are out of luck. I don’t know what to do, they are turning our service off Monday if the balance isn’t paid. Edit-added details about package weight