USPS transportation operations changes may affect the Postmark date of your tax related correspondence/payments. As such, you want to ensure your deadline sensitive mail is submitted earlier, or ideally, you should take it to the USPS counter and request a hand-stamped postmark showing the actual mailing date. Why this matters? IRS & other agencies use the postmark date to determine timely filing. A delayed postmark could lead to late penalties, interest charges or even rejected filings.
Postmarks will continue to be applied at USPS processing facilities; however, adjustments to USPS transportation operations may result in some mailpieces not arriving at USPS originating processing facilities on the same day that they are mailed. This means that the date on the postmarks applied at USPS processing facilties may not necessarily match the date on which the mailpiece was collected by a letter carrier or dropped off at a retail location, including via a collection box.
Note: If you wish to have your mail postmarked for the purpose of indicating the exact date it was submitted for mailing, you can go to any USPS retail counter and request a postal employee to manually postmark your envelope. (limitations do apply if sending more than 50 items)
Please review the USPS website for full details:
https://faq.usps.com/s/article/Requirements-and-Standards-for-Sending-Domestic-Mail…#postmarks