Getting scammed on a trade-in

I completed my trade-in within a week of getting the return label sent to me by Xfinity and am pretty much scammed out of $700. USPS tracking for the device shows it was lost since November and they refuse to approve a claim on the device. Xfinity has had multiple open tickets with me regarding this but they tell me to either make a claim with USPS, or reach out to Assurant. Assurant keeps claiming they have zero involvement with Xfinitys trade-in process.

Because Xfinity provided a return label with no insurance, it's their responsibility. Had I shipped this with my own label, I would have gotten the insurance and I wouldn't be dealing with this issue. Because all of Xfinitys support is overseas, it's worse than talking to a 3 year old, legitimately the worst customer service any company can offer. If it's not resolved within a week, I will have to take my mobile lines elsewhere, and take legal action, I have plenty of chats and documentation to do so.