Feif Un:
Well I just saw your post, better late than never, I guess. I definitely hear what you’re saying, but I don’t agree. A rental is something done for ones own benefit. She is not making any money from loaning the shtick, she is doing it as a chessed. To make the mitzvah even bigger, (and I’m assuming also so her gemach is not totally abused), she takes something for tzedakah. She is still providing a service which she is making no money on for the sake of the chessed. That to me is a gemach, even if she does take something for tzedakah.
(As a side point, I’m not sure if the tzedakah part is mandatory or optional)