Click Void next to the label (on Shipping Labels or on the Delivery Report's Shippo route) and confirm "Are you sure you want to void this shipping label?". Sprwt asks Shippo to cancel the transaction; on success the shipment is marked VOIDED (red badge) and the row goes back to offering Generate Label / Create Label so you can buy a new one. If the carrier refuses the void (for example after the package was scanned) the error returned by Shippo is shown and the label stays active.