New USPS Rule Affects Mailed Tax Payments, Donations

The U.S. Postal Service’s new final rule, effective on Dec. 24, 2025, serves as a timely reminder to request a same-day postmark when mailing tax payments and donations before the end of the year.

The rule doesn’t change any existing USPS postmarking procedures. Rather, it clarifies that a postmark simply indicates that the agency had your mailing on the date inscribed. A postmark date is not necessarily the date you dropped off your mailing. It may not even be the date the USPS first accepted your mailing.

That fact can present problems for individuals sending mail close to a deadline. For one, the IRS and state tax authorities depend on the postmark date to determine whether a mailed tax payment or return has been timely paid or filed. Likewise, charitable contributions sent through the mail are treated as made on the postmark date—it’s what’s called the “mailbox rule.”

So, even if you drop off your mail by the Dec. 31, 2025, deadline, the USPS may not postmark it until the new year. It’s best to have estimated state income tax payments postmarked by Dec. 31 to prove mailing within the calendar year for which you are claiming the deduction. For donations, your charitable contribution deduction may be delayed by a full year if you miss the Dec. 31 cut-off—and due to a tax law change taking effect in 2026, that donation deduction will also be reduced by 0.5% of adjusted gross income for individuals who itemize or 1% of taxable income for corporations.

There is something you can do to avoid a late-dated postmark. The new rule says that you can request a manual, same-day postmark at any U.S. Post Office when you drop off your mail (for free). You may have to wait in line, but it’s the best way to ensure that the envelope containing your payment, filing, or donation is postmarked in 2025.

Smith + Howard advisors generally recommend electronic filing and payments whenever possible. Not only are electronic payments generally safer, but they also help you avoid the potential consequences of a late-dated postmark. However, if mail can’t be avoided, it’s best to request a manual postmark or send your envelope by certified mail.