You run a prep floor. Inventory arrives from a dozen sellers in a dozen states of readiness, and it leaves polybagged, labelled and compliant on a schedule that Amazon sets and you absorb. Your margin is in throughput, so the things that cost you are the things that stop a station: a case that runs out mid-shift, a bag whose seal behaves differently from the last lot, a size you have to substitute because the right one is three days out.
Most suppliers sell you a case. What you actually need is a supply — the same spec, drawn against continuously, at a price that reflects the fact that you will buy it every month for as long as you are in business.