What must I do to rid myself of the consequences of using coupons that I obtained through illicit means, knowing that I have repented to Allah?

These coupons are not permissible unless the goods are declared with their usual price, and the buyer's intention is to purchase the goods, not the coupons. As long as you have not purchased, it is not lawful for you to take the coupons, and you must seek absolution from the shopkeepers or return the money to them.