What should I do if I mailed my income tax return and forgot to include the check? We encourage all taxpayers to pay electronically on our website whenever possible. If you prefer to pay the amount you owe on your Form IL-1040, Individual Income Tax Return, by mail, complete
Form IL-1040-V and send it, along with your payment, to the address on the voucher. Do not send cash. If you have received a bill, please follow the instructions on the bill.