When you void a label within SKULabs, SKULabs submits a refund request for the postage, label creation price, and any insurance purchased through your carrier.
Each carrier has their own policies regarding refunds. FedEx and UPS labels are typically bill-on-scan but confirm with your representative. USPS labels must usually be voided within 24 hours of creation and take 30 days before they are refunded–usually as account credit with whichever USPS integration partner you're using.
Here is a helpful article from Shippo: