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After you cancel a USPS label, it takes Endicia about 10 days to verify that the label did not show up in the mailstream. This is mentioned in the following article from our Knowledge Base: How long does it take to receive a refund after cancelling a USPS label?
You can track your Endicia transactions and refund requests by following the process outlined here: How to: Obtain my Endicia refund and transaction history. Please keep in mind you have two Endicia accounts so you will need to be sure to log into the corresponding one.
For future shipments, you might consider Endicia's Pay-on-use returns otherwise known as Scan-based returns. There is a small fee for those labels but the shipment cost does not come out of your account unless the return label is scanned. You can see more about this feature here: FAQs: Scan-based return shipments