FWIW I have purchased a Shippo /USPS label and for a transaction that I had not used a Shippo label on.
I had, as usual, used stamps and bought the Tracked Packet label at the PO.
When the buyer wanted a refund, I went to Shippo, and bought as a new purchase unrelated to any previous transaction , a USPS label.
So.
Try it that way. Don't relate your purchase to the original shipment.
It might not work, in which case you are no worse off.
And if it does work, put the tracking number in the Dispute.
https://postcalc.usps.com/Calculator/Index