Even if they accurately charged shelf prices, these places are still a ripoff targeting the vulnerable. The list price is low but the per-unit price is astronomical compared to grocery store prices.
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The problem is, so is material cost and handling effort. Say, a 2 liter bottle of soda compared to 10x 200 mL. Same amount of soda, but more handling required for stocking, inventory management (aka, make sure there is no soda expiring on the shelf) and finally scanning it over the cash register, and more packaging material.
Larger units of anything will always be cheaper than small units.