Shippo automatically voids and refunds unused labels after 30 days for their USPS-master account and selected other accounts. See here for details https://support.goshippo.com/hc/en-us/articles/201772785-Refund-a-Label#h_aced7663-ca22-4ec2-88c5-05f1b82f400b
ShipStation requires to manually void labels inside the ShipStation dashboard within 28 days for their Stamps.com account. See here for details https://help.shipstation.com/hc/en-us/articles/360026157751-Void-Labels#UUID-997e8eed-f324-947f-f8c3-d80f909e18a6.
Shipping labels created through EasyPost are generally eligible for a refund before the label has been used to ship a package with the carrier. Refunds can be requested on the EasyPost dashboard Shipments Page, but they can also be refunded via the API Shipment Refunds method as documented in their API Documentation. In order to refund a shipment, follow the steps outlined in the following article: https://support.easypost.com/hc/en-us/articles/360042414392-Refunding-Labels.