72 Pa. Stat. § 7403.1
Notwithstanding the provisions of any State tax law to the contrary, whenever a report or payment of all or any portion of a State tax is required by law to be received by the Pennsylvania Department of Revenue or other agency of the Commonwealth on or before a day certain, the corporation shall be deemed to have complied with such law if the letter transmitting the report or payment of such tax which has been received by the department is postmarked by the United States Postal Service on or prior to the final day on which the payment is to be received.
For the purposes of this article, presentation of a receipt indicating that the report or payment was mailed by registered or certified mail on or before the due date shall be evidence of timely filing and payment.
72 P.S. § 7403.1