i know this is not shoulder-carried, but is the manhattan PM heavy?
^^^ yes the Manhattan PM is somewhat heavy because of the hardware.
May I suggest the Beaubourg? I know it looks a little masculine, but it's lightweight and can certainly carry a little or a lot. You can accentuate it with a charm or bandeau to make it a bit feminine. It has a wide open compartment with cell slot and a zippered compartment for your valuables. I've attached pics for you
