Answer: The customer will need to be shown the coupon where it states that the coupon is only valid for a full-price item and not a sale item. Every coupon usually has this stated on the front lower corner, or the back lower corner of the coupon. Always be nice to the customer when explaining why she can't use the coupon at that time. You can explain to the customer that the product is still on sale and she is saving money. If the customer still doesn't understand and is getting angry, you can request a manager to come and speak to the customer.