The 2026 USPS postmark rule change significantly alters how tax returns are considered filed, as they will only be officially postmarked at USPS processing facilities. This means that returns mailed on April 15 could be deemed late if there are any delays in collection or processing, leading to potential IRS penalties. The late filing penalty starts at 5% of unpaid taxes per month, capped at 25%, while additional penalties for late payments can also apply. Taxpayers are urged to reconsider last-minute mailing strategies, as even a short delay can result in severe financial consequences. To mitigate risks, filing electronically is recommended, but if mailing is necessary, obtaining a hand-cancel or round-date stamp at a USPS counter can provide proof of timely mailing. Taxpayers should also consider mailing their returns several days in advance to avoid the uncertainties introduced by the new rules.