After a shipment has been voided or canceled via the POST Shipment Refund endpoint, you may be eligible for a refund of the shipping costs.
For carriers that are associated with funds in a PackageHub account, refunds will be credited back to the account for unused labels after a successful refund request.
A refund may be rejected if the label has been scanned by a carrier, the shipment is in transit, the shipment has been delivered, or the label is past the carrier's refund eligibility period.
For additional details, see USPS Refund Processing Time