If you buy shampoo that is $1.20 with a $2 coupon you would have .80 overage. If the store only takes off the $1.20 I would be sure that they write $1.20 on the coupon because that is fraud. That means the store profited .80 if they collect the $2. from the manufacturer.
I always write out my own coupons if the store doesnt give me the money on store credit.