So much depends on how often you're with this person & how much you interact.
First, you mentioned shadowing....which to me implies just watching. I couldn't really write you a letter if you just watched what I did - even if you spent years just watching.
Then, you mentioned volunteering...which to me is a totally different thing. That is more in the realm of "work" - I ask you to do something, you may or may not know how, I show you, you do it, then in my mind, I think - good - he/she did a good job or aaargh - I'll never ask that of him/her again or perhaps I need to reteach him/her.
Now....if you're actually doing tasks and if you are coming often enough - say at least once a week, then I'll get an insight into how you handle work which is given to you. I'll get an idea of how willing you are to do unfamiliar things, how pleasant you are, how well you take direction, how reliable you are.
The issue here is - if you volunteer for years, but only come once a month...you never learn anything well enough so I know I can rely on you to do it (or....I may have forgotten I even taught you how to do it - I'm old
!) However, if you come weekly or more than once weekly, for just a few months....I'll get some idea of your personality, your work ethic, how distractable you are, how realiable you are, how interested you are.
Does that help you know what you'll need to give the pharmacist so you can get a good LOR???
Good luck!