I misunderstood your issue - you're saying that the marketing text isn't excluded (it's not a discounting issue, it's a marketing text issue).
The answer is, you have to customize other things if you take advantage of the user exits. There's an example in the help of how to do this.
https://www.thatsoftwareguy.com/osc_quantity_discounts.html#marketing
Go down to the second note below this link.