Walmart is the biggest purchaser of many goods which puts it as the counterpart to a monopoly; a monopsony (single purchaser instead of single producer).

Walmart has incredible negotiating power, when there is a price shock Walmart forces its suppliers to take the hit with the threat of never buying from them again; which it can do. One of the efficiencies of Walmart’s economy of scale is that it pays below market rate for most of its goods because of bull purchase efficiencies and negotiation power.

https://www.econ.ucdavis.edu/events/papers/1014JustinCWiltshire_JMP.pdf