The reason the conventional stores do quantity purchases is because they are competing with the "stock the pantry" Club Stores! If you haven't noticed, Wal-Mart is now stocking pack sizes you normally see in Sam's.
Usually, the deals you see on basic grocery items are handled by "bill back" through the conpany Ad Management programs. The Ad Management DSD leads negotiate the costs with all DSD (Frito Lay, Coke, Pepsi, Beer, Etc...)
Very little markup in those lines at store level because they have built in labor cost for distribution and service.
As for the Air in the bags, think about the small single serve bags at the sandwich shops. I always get Cheetos because you get an almost full bag. Chips are so thin, they settle in the bags from the auto filler to the store. What's worse is the distribution rep trying to get as many bags on the shelf as possible and jamming the chip bags crushing the chips. That ticks my wife off way more then the air in bags