35 ILCS 115/17
If it shall appear that an amount of tax or penalty or interest has been paid in error hereunder directly to the Department by a serviceman, whether such amount be paid through a mistake of fact or an error of law, such serviceman may file a claim for credit or refund with the Department. If it shall appear that an amount of tax or penalty or interest has been paid in error to the Department hereunder by a supplier who is required or authorized to collect and remit the Service Occupation Tax, whether such amount be paid through a mistake of fact or an error of law, such supplier may file a claim for credit or refund with the Department, provided that no credit shall be allowed nor any refund made for any amount paid by any such supplier unless it shall appear that he bore the burden of such amount and did not shift the burden thereof to anyone else (as in the case of a duplicated tax payment which the supplier made to the Department and did not collect from anyone else), or unless it shall appear that he or his legal representative has unconditionally repaid such amount to his vendee (1) who bore the burden thereof and has not shifted such burden directly or indirectly in any manner whatsoever; (2) who, if he has shifted such burden, has repaid unconditionally such amount to his own vendee, and (3) who is not entitled to receive any reimbursement therefor from any other source than from his supplier, nor to be relieved of such burden in any other manner whatsoever.
Any credit or refund that is allowed under this Section shall bear interest at the rate and in the manner specified in the Uniform Penalty and Interest Act.
Any claim filed hereunder shall be filed upon a form prescribed and furnished by the Department. The claim shall be signed by the claimant (or by the claimant's legal representative if the claimant shall have died or become a person under legal disability), or by a duly authorized agent of the claimant or his or her legal representative.
A claim for credit or refund shall be considered to have been filed with the Department on the date upon which it is received by the Department. Upon receipt of any claim for credit or refund filed under this Act, any officer or employee of the Department, authorized in writing by the Director of Revenue to acknowledge receipt of such claims on behalf of the Department, shall execute on behalf of the Department, and shall deliver or mail to the claimant or his or her duly authorized agent, a written receipt, acknowledging that the claim has been filed with the Department, describing the claim in sufficient detail to identify it and stating the date upon which the claim was received by the Department. Such written receipt shall be prima facie evidence that the Department received the claim described in such receipt and shall be prima facie evidence of the date when such claim was received by the Department. In the absence of such a written receipt, the records of the Department as to when the claim was received by the Department, or as to whether or not the claim was received at all by the Department, shall be deemed to be prima facie correct upon these questions in the event of any dispute between the claimant (or his legal representative) and the Department concerning these questions.
In case the Department determines that the claimant is entitled to a refund, such refund shall be made only from the Aviation Fuel Sales Tax Refund Fund or from such appropriation as may be available for that purpose, as appropriate. If it appears unlikely that the amount available would permit everyone having a claim allowed during the period covered by such appropriation or from the Aviation Fuel Sales Tax Refund Fund, as appropriate, to elect to receive a cash refund, the Department, by rule or regulation, shall provide for the payment of refunds in hardship cases and shall define what types of cases qualify as hardship cases.
35 ILCS 115/17