72 Pa. Stat. § 9166
.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 department or other agency of the Commonwealth on or before a day certain, the taxpayer shall be deemed to have complied with the law if the letter transmitting the report or payment of the 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. Any inheritance tax return filed after July 1, 2012, under section 2136 that reports transfers of property that are exempt from the inheritance tax under section 2111(s), (s.1) and (t) shall be considered timely filed if filed within one year of the tax return due date, including an extended due date.
72 P.S. § 9166