problem with claim on AtoZ and no protection

I purchased shipping label on amazon.com

shipped same day, used fedex/usps shipping, fedex tracking showing nothing, usps tracking is showing that shipping label was not paid for and they refuse to deliver or to return this package.

i told buyer to open atoz claim to get reimbursement, they did, atoz took my money and reibmursed them, i appealed and gave they usps tracking part, they refused to reimburse us.

Still have a copy of the original shipping label that I paid for, and can not submit this to them.

opened case 17485936661 to ask for help, they will just transfer.

PLEASE HELP HERE.