Section 66.74 - Payment or reimbursement(a) Within thirty days after any adjustment of a noncompliance penalty under this subpart has become administratively final:(1) Any deficiency owed by the source owner or operator shall be paid as provided in § 66.62 .(2) Any reimbursement shall be paid by check from the United States payable to the order of the source owner or operator, and sent by registered or certified mail, return receipt requested.(b) Any payment under paragraph (a) of this section, shall include interest on the amount of the deficiency or reimbursement due, from the date the deficiency or reimbursement arose, at a rate determined by the Secretary of the Treasury. Such payment shall be calculated in accordance with the Technical Support Document and the Manual.(c) Any source owner or operator who fails to make timely payment of a deficiency shall pay a nonpayment penalty. The nonpayment penalty shall be calculated as of the due date of the deficiency payment and shall be equal to 20% of the deficiency not paid. Such nonpayment penalty (in addition to the amount of the deficiency owed) shall be payable immediately. If any part of the nonpayment penalty or deficiency shall remain unpaid at the end of three calendar months from the due date of the deficiency, a further nonpayment penalty shall be due equal to 20% of the sum of all payments due and owing. Partial payments shall be credited first against the nonpayment penalty, then the deficiency.