Ill. Admin. Code tit. 35, § 270.606 – Refund and Underpayment of Fees

a) A fee remitted to the Agency under this Subpart shall not be refunded in whole or part unless the amount paid is in excess of the amount billed or the amount billed is determined to be incorrect.
b) An owner or operator of a source entitled to a refund under subsection (a) above must request such refund in writing; otherwise, the amount that was overpaid incorrectly shall be credited against future fees for which the source will be billed under this Subpart.
c) If the owner or operator of a source submits to the Agency an amount less that the amount due pursuant to Section 39.5(18) of the Act and this Subpart due to an error in the source's calculation of allowable emissions or an error in the Agency's calculation of fees, the owner or operator shall submit the difference between the amount paid and the amount due in accordance with subsections (1) or (2) below.
1. 1) If a source discovers that its calculation of allowable emissions was incorrect, it shall promptly submit to the Agency the correct information and the amount of underpayment.
2. 2) If the Agency discovers that it has incorrectly billed the source, the Agency shall promptly notify the owner or operator of the CAAPP source. The Agency must notify the owner or operator of the incorrect billing during the term of the permit. The owner or operator of the CAAPP source shall submit the amount of underpayment within 45 days after receipt of the notice.