35 ILCS 120/2-40
If a seller collects an amount (however designated) that purports to reimburse the seller for retailers' occupation tax liability measured by receipts that are not subject to retailers' occupation tax, or if a seller, in collecting an amount (however designated) that purports to reimburse the seller for retailers' occupation tax liability measured by receipts that are subject to tax under this Act, collects more from the purchaser than the seller's retailers' occupation tax liability on the transaction, the purchaser shall have a legal right to claim a refund of that amount from the seller. If, however, that amount is not refunded to the purchaser for any reason, the seller is liable to pay that amount to the Department. This paragraph does not apply to an amount collected by the seller as reimbursement for the seller's retailers' occupation tax liability on receipts that are subject to tax under this Act as long as the collection is made in compliance with the tax collection brackets prescribed by the Department in its rules and regulations.
35 ILCS 120/2-40