No refund

I bought a hydration pack from Due South online because there was a 20% online special. On trying out the bag out it was faulty. In the returns policy section it states you can return the product to any store and get a full refund. On returning it I was informed that my card would be refunded. I have now received the refund, but not the full amount. The delivery was not refunded. As it states in the terms and conditions that there would be a full refund, and nothing about deducting the delivery I expect to be refunded the full amount. Especially since it was their product that was faulty. If the product worked I would not have had to return it. Why should I have to pay for them making faulty products. I expect my full refund as stated in the terms and conditions. I am very disappointed at this service as I often buy things online from other companies and sometimes need to exchange and return and always get very good service. The company is liable for delivery charges if they sell faulty products online. I have never before heard of being charged for delivery f a faulty product.