Cool story time... I used to deal with shipping TO Walmart.
Utter fucking bastards, that's what they are.
Ensure all products are re-labeled to their standard instead of UPCs. Ensure all products are re-boxed according to their standard. Ship only on CHEP pallets (I sympathize with this one, actually, dealing with decaying pallets and skids isn't worth it).
Delivery? Well, any company you want, but they must arrive at a specific time and then wait upon Walmart's pleasure. Even if Walmart receiving has lined up 10x more trucks than they can process. If your delivery driver gives up and leaves, that's a missed delivery and a penalty... so when you ship you hope to God for a morning appointment and then further hope you won't have to pay the driver to sit outside their gates for the full day.
Then the inspection. If anything, and I mean ANYTHING isn't 100% correct, there's a good chance they'll turn back the whole shipment. And assess a penalty.
But hey, Walmart says they'll buy from you if you cut into your margins, and they'll buy 100x what you normally move, so it looks really, really attractive. Eventually you find you're sweating to death working for pennies and now your company is basically toast if you stop. THAT is when Walmart demands another cut in price, and if you can't do it, they just find someone else and you watch your company crash around you.