There is a checkbox under Payment Methods when you go to the purchase page. If you click it, you will receive a $10 discount for having purchased PT2/3 on your purchase of PTO.
If you already own PT2 Hold'em then you get the discount on PTO, I'm not sure why that option isn't available right now. Just check the box that you already own PTS and the discount will be applied. We'll figure it out when we verify that you already have PTH.