When a provider lists a price, say 150, how much tip should one provide?  Do you include it in the donation envelope or do you leave it on your way out the door at the of the session?

I'd say $20 to whatever you want. Remember, you are setting a precedent for future visits. I never know if I want to tip in any service industry until I receive service.
Envelope is fine, or on a table etc, after you brikng it to her attn...I had a tip left once I dind't even see, and thank goodness my neighbor's child is honest!!