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No need for that if you load the trolley or basket correctly in the first place. Then scan the heavy items first (which will have been piled up on one side of the basket/trolley so as to not crush the light stuff during the shop).
And I've yet to find a self service till that hasn't properly recognised my own bags, or if I've gone without one, had a problem scanning a new bag.
Segregate the tills, I say. Those who know how to maximise efficiency get their own lane.
Supermarkets are soul destroying places to be, why would you willingly prolong the time spent in one?
Scan as you shop is the best thing to happen to supermarket shopping. Bonus points if you pack your stuff on the way round with a thought of how it needs to be put away at home.
Makes for more rational bag packing anyway. Place heavy items in one section of the bagging area, lighter / crushable items in another. Heavy items in the bags first, even distribution of weight between bags.