69 Pa. Stat. § 621
A default charge may be collected on any installment payment or payments which are not paid on or before the due date of such payments. A default charge may be collected on any contract subject to this act, regardless of the class of vehicle as defined in section 19 or the method by which the finance charge is computed. Such default charge shall not exceed the rate of two percent (2%) per month on the amount of the payment or payments in arrears where the contract is for the sale of a motor vehicle which is other than a heavy commercial motor vehicle. Such default charge may be computed on the basis of a full calendar month for any fractional month period in excess of ten (10) days. On any contract for the sale of a heavy commercial motor vehicle, the default charge shall not exceed for any payment not made within ten (10) days of its scheduled due date, four percent (4%) of the amount of the payment or payments in arrears: Provided, That such default charge may be collected only once on each payment in arrears. Such default charges may be collected, when earned, during the term of the contract, or may be accumulated and collected at final maturity, or at the time of final payment under the contract. Such default charge shall not be collected on any payment in default because of any acceleration provision in the contract.
69 P.S. § 621