@ahsaaful I'm not sure but I think it's normal because the ordering process needs to be completed and cannot be completed without paying even if you don't have to pay. Does that even make sense?
If the balance is zero after you entered your coupon I assume your creditcard or paypal won't be charged but it's just a needed step to complete your order.
I suggest contacting Yesstyle/Yesasia to be sure. They have facebook