75 Pa. C.S. § 9617
With respect to all reports, claims, statements and other documents required to be filed and all payments required to be made under this chapter, any such report, claim, statement and other document or payment of tax withheld shall be considered as timely filed if the report, claim, statement or other document or payment which has been received by the department is postmarked by the United States Postal Service on or prior to the final day on which payment is to be received. For the purposes of this chapter, the presentation of a receipt indicating that the report, claim, statement or other document or payment was mailed by registered or certified mail on or before the due date shall be prima facie evidence of timely filing of the report, claim, statement or other document or payment.
75 Pa.C.S. § 9617