You have 30 days from the date you created the label to cancel it, which automatically triggers a request for an eRefund.
Once you cancel a label that was generated through Endicia it can take 14 to 24 days to process the refund. USPS requires 10 business days to process the postage refund on their end, as they need to ensure that the tracking number for the cancelled label doesn't subsequently show up in the mail stream. So, Endicia will wait 10 days to begin processing the refund. It then takes Endicia another 10-14 days to finalize the refund.
Within 24 days from cancelling a shipment, the shipping paid will be refunded back to your postage balance.
For additional information about obtaining proof of Endicia refunds, click here.