> How is it that a small grocery store can sometimes provide things (much) more cheaply than a megacorporation?

Megacorps have overheads for managing insane supply lines and lots more stringent enforcement with their product, and generally have much more oversight for things like labor.

The tiny grocery store uses the kid as free labor after school to keep costs down (I was one of those kids), and generally cares less about the quality of the product at the individual level.