When you juice fast you should ONLY have juice and water. Nothing else. I've never read a resource of any kind that says it's o.k. to add P&B shakes to fasting. The maker's of P&B don't even say it's o.k. to do (they actually don't have a stand).
They are o.k. to do while you're not fasting, but while fasting you don't need them.
Oh, and when/if you stop the shakes, don't be surprised if you start getting hungry again. hth :)