35 ILCS 173/5-35
Each delivering supplier who is required under Section 5-15 to collect the tax imposed by this Law shall make a return to the Department on or before the 15th day of each month for the preceding calendar month stating the following:
In making such return the person engaged in the business of delivering gas to persons for use or consumption and not for resale may use any reasonable method to derive reportable "therms" from his or her billing and payment records.
If the average monthly liability to the Department of the delivering supplier does not exceed $100, the Department may authorize his or her returns to be filed on a quarter-annual basis, with the return for January, February, and March of a given year being due by April 30 of such year; with the return for April, May, and June of a given year being due by July 31 of such year; with the return for July, August, and September of a given year being due by October 31 of such year; and with the return for October, November, and December of a given year being due by January 31 of the following year.
If the average monthly liability to the Department of the delivering supplier does not exceed $20, the Department may authorize his or her returns to be filed on an annual basis, with the return for a given year being due by January 31 of the following year.
Such quarter-annual and annual returns, as to form and substance, shall be subject to the same requirements as monthly returns.
Notwithstanding any other provision in this Law concerning the time within which a delivering supplier may file his or her return, in the case of any delivering supplier who ceases to engage in a kind of business that makes him or her responsible for filing returns under this Law, such delivering supplier shall file a final return under this Law with the Department not more than one month after discontinuing such business.
Each delivering supplier whose average monthly liability to the Department under this Law was $10,000 or more during the preceding calendar year, excluding the month of highest liability and the month of lowest liability in such calendar year, and who is not operated by a unit of local government, shall make estimated payments to the Department on or before the 7th, 15th, 22nd, and last day of the month during which tax liability to the Department is incurred in an amount not less than the lower of either 22.5% of such person's actual tax liability for the month or 25% of such person's actual tax liability for the same calendar month of the preceding year. The amount of such quarter-monthly payments shall be credited against the final tax liability of such person's return for that month. Any outstanding credit, approved by the Department, arising from such person's overpayment of his or her final tax liability for any month may be applied to reduce the amount of any subsequent quarter-monthly payment or credited against the final tax liability of such person's return for any subsequent month. If any quarter-monthly payment is not paid at the time or in the amount required by this Section, such person shall be liable for penalty and interest on the difference between the minimum amount due as a payment and the amount of such payment actually and timely paid, except insofar as such person has previously made payments for that month to the Department in excess of the minimum payments previously due.
The delivering supplier making the return provided for in this Section shall, at the time of making such return, pay to the Department the amount of tax imposed by this Law. All moneys received by the Department under this Law shall be paid into the General Revenue Fund in the State treasury.
35 ILCS 173/5-35