To cancel postage (and request a refund) navigate to the History page. There you can locate all previously generated postage. Locate the label, and click the Cancel and Request Refund button.
Postcards / First Class letters / Flats can not be cancelled / refunded according to USPS policies.
Please note: Postage is generally refunded within 1 business day. However, this can take up to 30 days, as The USPS is notified and there may be times when the postage is accidentally used, or the USPS notifies us of an issue.