As long as a shipment's label hasn't been used at all, you will see it in "Pre-Transit" status in the shipping list. Click the ellipse (...) menu to the right of the shipment and select "Request Refund." Enter the information about why you're making the request and submit.
Our CS team will review the request and let you know as soon as it's been processed. The label will be cancelled and the amount paid is returned back into your shipping account.
Labels that have been refunded cannot be subsequently used; if they are, USPS will charge the label as an assessorial charge and your shipping account will be debited for that label.