I'm not sure if you can take the label to your local Post Office (not postal outlet) and ask about upgrading on your own dime.
The cost , if even possible, would be under $5. You want a Tracked Parcel USA label.
It is possible that the seller did not know the difference. I ship to the USA every day, but know almost nothing about USPS standards and rates.
Without tracking the sender (which is you in this case) cannot electronically prove return delivery.
I hope your PO clerks are in a good mood.