61 Pa. Code § 313.14
Every motor carrier shall be prepared to present evidence to substantiate the credit claimed for payment of the liquid fuels tax on motor fuels purchased within this Commonwealth. Such evidence shall consist of invoices of the vendor of the motor fuel which shall show the name and address of the motor carrier, the point of delivery, the date of each sale, the number of gallons of each sale, the total monetary value of each sale, and the license number or unit number of the motor vehicle being fueled. Sales made out to cash are not acceptable. The Department will have the right in order to further substantiate the credit claimed, to require a sworn affidavit from the vendor stating that it has sold the specified number of gallons of motor fuel to the motor carrier and that the liquid fuels tax has been paid on such motor fuel.
61 Pa. Code § 313.14