Discount item is supported with Salesforce - NetSuite v2 Integration Application.
If the discount item doesn’t sync, you have to check the number format on the rate of the item. It should have a positive number with percentage “%” appended to the number. Adding a negative whole number or just a whole number without a “%” will not sync the record.
Example: Below numbers in the rate field will not sync the discount item
Example: Below number in the rate field will sync the discount item