Trading in mobile phone

This transaction was handled online by me for my mom. We have a plan together on Xfinity mobile. I traded in her iPhone 13 for an iPhone 15. When the phone came, it was a nightmare getting it up and running and activated when I finally received a label to return the iPhone 13 for credit. The label was not printable so I had to go on chats lots of chats and it was never rectified. I kept getting put onto different people then getting different phone numbers everyone promised it would be handled finally I got a printable label from UPS brought the phone in and mailed it received confirmation that it was received and that it had to be checked to make sure it was in good condition. Then I received an email saying the phone was in good condition. I would receive my $143 credit then I went on many many chats again to receive the credit was promised credit never got the credit. I spent so much time on this because it got to be a matter of principal. And I still haven’t gotten any credit. I’ve been working on this since October.