Hi team,
I read the documents relevant to order invoice. there are 2 objects called shipping_amount_after_discount and shipping_discount_amount. I wonder if it’s right in the logic that
shipping_amount_after_discount= original_shipping_fee –  shipping_discount_amount.
and 
shipping_amount_after_discount would be the final shipping fee  that a customer will pay. 
if yes, where can I get the original shipping fees before the customer applied the discount amount?
Can you help me to clarify this? Thank you all ^^