The declared value of your order is the order subtotal. This means that discounts are included in it. However, Rewards credits are not discounts and considered to be a payment method, so they are included in the declared value.
We would advise using the Subtotal in the cart to estimate the Declared Value for the order. We recommend ordering less than the allowable importing amount before any discounts are applied to make sure that your order is not over the allowed limits and to avoid customs fees.
Unfortunately, our system will not allow adjusting the payments records to be less than the Customs amount allowed.