Xfinity Mobile Returned Device

I switched to Xfinity Mobile, I brought 2 phones and my wife wanted to go back to Samsung from iPhone, so she ordered a Galaxy A42. She used it for 2 days and decided she wanted to use her old iPhone instead. We contacted Xfinity Mobile for a return label and sent it back within 2 weeks total time… once the phone was received I saw a charge on my card for $$311.16 this was the balance of the full $350 price of which I paid $38.84 when ordered. After a couple of phone calls to find out why I was charged I was told because there was significant damage and it was rated a C. I advised them that the phone was only used for two days, and there was no damage. The representative then asked me. If I had taken a picture of the phone before, I put it in the box and mailed it back. I said, of course, not, and then I asked if they have a picture of the box they received to show that it was intact and not FedEx‘s fault. Long story short after multiple conversations with multiple supervisors, I was told that there is no way they can prove to me the condition of the phone only that it is rated a C. He apologized that I would have to pay for damages, which he doesn’t have details of or any proof of at all…. Is this the way Xfinity does business?