Either in the cart panel's Coupons row (pencil icon) or on the payment page under Coupons > Edit, the Add Coupon panel opens with a search box (Click here to search by coupon name) listing your coupons by name/code. Click a coupon to apply it; the discount appears in the totals and the coupon name is listed on the order. Invalid coupons show an error message under Coupons. The coupon controls require the List Coupons's permission.