Things like toothbrushes are locked up because they are relatively expensive but so small that they are easy to steal. You can just slide one up your sleeve while pretending to examine another one and then putting it back. It's not worth trying to identify or detain shoplifters over toothbrushes, but the costs of theft add up. Same thing with razor blades.
Used to work at a market as a grunt and later in mgmt doing orders. No supermarket orders pallets of blades. Cost per sqft to warehouse a pallet for years as they slowly sell or are stolen is too high.
If you're using it as a proxy for cost per volume then lately prices have exploded such that blades win, but the winners used to be pharmaceuticals, batteries, film (ye olden days), and oddly enough spices. Spices were good enough for galleons in the middle ages, so I guess its no great surprise. The price of a cardboard box full of genuine vanilla flavoring or maple syrup is fairly shocking if never considered before.
If you demand pallet level ordering / money laundering it is surely laundry detergent. Nothing else comes close in cost at the required pallet level sales volume. Certainly not $1/gallon distilled water pallets or $5 bags of road salt.
