I had the same issue when I was in sales. I had a perfectly good, big account ("S"), who suddenly decided that 4% gross profit margin was too high for them, even though that was bare bones for us. They demanded 1 1/2%. Well, cost of doing business was about 2%, as it was through most of the industry. I said no way.

Of course, they found some salesperson with dreams of having a big account in such a glamourous industry -- volume, volume, volume! One little shop after another either went out of business, or refused to sell to them after a while. None of them could really service the client or provide good quality at that price point. The end users at this big company were the ones that suffered. Eventually, said client started buying from me again at the regular pricing when they just couldn't get their 1 1/2%.

With a store like Walmart, there will always be another vendor or salesperson ready to take that sucker bet.