If you need to cancel a shipment and it has not been shipped yet by carrier, you could delete the shipment record created with A5 Ship app in Salesforce or the shipment status will remain “ready to ship” if you would like to keep the shipment record. It is only when you give the package to the carrier to be shipped that the carrier will scan and process the label so if they have not yet received package, you can “cancel” the label before shipment starts.
Exception to this process is USPS over Endicia. When you create a shipment record and label, you automatically pay the shipping. To cancel the shipment and get a refund, you can go to shipment record and click on Cancel Shipment (this button is not automatically displayed on Shipment page layout, so you have to add it to the layout first).
If the shipment has been shipped or in route to destination, you will need to contact the carrier directly to cancel the shipment.