To avoid paying penalty, your payment envelope must be postmarked on/or before the last day of the taxes' penalty-free collection period. By law, only an official U.S. Postal Service cancellation date stamp is acceptable as proof that your payment was mailed on time. A metered postmark is not acceptable by law. If making payment close to a tax deadline, we suggest that payment be sent by registered or certified mail, or have the envelope "hand" canceled with the date at your local post office.