Is that legal?
Here in Virginia (which isn't nearly, as, err, regulated as Illinois) a shop is allowed to charge a fee for accepting used oil but it's limited, and nowhere near $21.
One likely reason for limiting the amount a shop can charge for disposal is that charging such a high amount is likely to increase the incidence of improper (down the storm drain) disposal of used oil.
Here in Virginia (which isn't nearly, as, err, regulated as Illinois) a shop is allowed to charge a fee for accepting used oil but it's limited, and nowhere near $21.
One likely reason for limiting the amount a shop can charge for disposal is that charging such a high amount is likely to increase the incidence of improper (down the storm drain) disposal of used oil.