I have bought a new bag which I can return- it's a Modalu grab bag.
I do find that shoulder bags although they leave your hands free, give you poor posture and I do have a dropped shoulder now from carrying bags.
I've got a Radley messenger across the body bag which I use quite a bit but it doesn't hold much - phone, keys, lipstick, purse and that's it.
Somehow i can't help feeling like the queen with a double strap grab bag- so convince me otherwise please!