Phd Student here: Where is the line between free labor abuse vs....whatever the "good" version of free labor is? I have an advisor who is very supportive. She responds quickly, offers advice and helpful edits (often on a short turnaround from my own delays), and seems invested in my doing well in the program (Social Sciences program in America). 
However, I have, over the years, come to observe that she is prone to seeing others for ways she can "use" them, and has displayed at times disrespect for others' time, etc. So that's made me put my own guard up to look for the same treatment of me. 
She helped facilitate my funding for the year (but it's funding for me to do work for another agency), so I think strictly speaking, I owe that agency 20 hours/week, and my other time is meant for degree progress (part of which is receiving mentorship from advisor). But I know that sometimes, much of my week goes towards helping my advisor with her stuff. Her grants, projects, pilot data, administrative work, journal article reviews, etc. The most agitating are absolutely menial tasks like picking up her mail and moving it from one building to another. 

So I am wondering...at what point am I just a great advisee, with whom she produces some good work, and for which she will be a long-term (career-long??) mentor and friend and recommender... and at what point am I just being used for free labor? 

Edit: The mail pickup is not just junk magazine/letters..but packages and such, some of which may be for my specific studies. But there have been times when, even for packages that are NOT for my studies, I've been asked to do X,Y,Z with them.
I am very sympathetic to your supervisor, so my discussion here will be biased. Good supervisors are not given much recognition nor appreciated much by the system. The system seems to reward poor supervisors by giving them time to do more research. 

Could this be a quid-pro-quo situation? It is clear that menial tasks are out of line, and there is a line between exploitation and also asking for assistance. Especially, so she can spend more time to do more for you and her students, providing responsive feedback etc. So the issue is whether you feel appreciated enough right now? And how much time you are spending for her and her agendas. How much of her agenda lines up with your future goals and aspirations? Try to align your agenda and activities so you are not feeling exploited but part of her team. 

Despite all this, if your agenda is not aligning, then speak up and try to set boundaries. Realise that your time and access may deteriorate to the bare minimum that other supervisors seem to give? May not necessarily be as retaliation but rather as the result of her time being reduced as well for "menial tasks". Some people really struggle with "menial tasks" and changing focus can be a struggle for those people who prefer conceptual and more abstract work.