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.

For refunds via EasyPost or other carriers on EasyPost, they have the best article on that topic.

Billing - Frequently Asked Questions - How does billing work from each carrier at EasyPost?

Refunds - Frequently Asked Questions
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It takes about 30 days to get the refund for a USPS label.
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Here is a helpful article for refunding labels from Shippo:
https://support.goshippo.com/hc/en-us/articles/201772785-Refund-a-Label