A discount item can be used to reduce the total amount of a transaction.
To apply a discount item to a transaction’s total, select the discount item you want to apply in the Discount field. NetSuite auto-fills the item’s rate and the transaction’s discounted total.
In this case, the Discount is equally applied to all line level Items. To omit certain items from the Discount getting applied, we could use a Discount type item below the CUSTOM item where the discount should be ignored. This Discount item should be added with a positive amount, reversing the discount applied.