Glad to know I'm not the only one carting around with groceries on the floorboard. I'm not sure whether it's legal, but every now and then I'm riding about 5km with.......believe it or not - a 7kg bag of potato's lying on it's side on the floor board, with my feet on top of it.
As for stability, I'm not too sure - the riding position with a bag of potato's under the feet is a bit awkward. I usually concentrate not to drop the potato's or scooter, therefore I haven't noticed a difference in stability so far.