Why is volunteering important to you? Is this the only hospital for miles or is there somewhere that could use your help on weekends or something like that?
Your PI employed you and expects you to be there. You want to something on her time that, if you must do it, you should be doing on your own time.
If you are coming into lab on your own time, that's nice but it is a gift to her of your time because you don't owe her that time and she doesn't own you. However, you are expecting some quid pro quo, that you give her some weekend hours and she gives you the late morning one day a week... but no one told her that was the deal.
She wants you there and you are telling her,"I don't want to be here, I want to do what I want to do because your work and your lab is not important to me and I am focusing my efforts on moving on out.... " After saying that with your actions, do you expect to get a good letter?