Oh, that's easy to get around. BigStoreSupply sells the tablet to BigStore under cost, so BigStore can sell it at a technical markup. BigStoreSupply is legally a different company, in a different state. BigStore can say 'See, here's our receipts, we bought it for $22.50 and sold it for $29!' because BigStoreSupply is technically not the same corporation as BigStore.
State: "Well, how much did BigStoreSupply pay for it?" BigStore: "Oh, I have no idea. You'd have to ask them." BigStoreSupply: "We're not in your jurisdiction, we don't have to tell you what we paid for our products."
Although with Wal-Mart, it's more likely 'Hi. you're going to sell us a crapload of tablets REALLY CHEAP so we can put them on special, or we won't carry your junk any more.' ''kay.'
That doesn't mean Wally normally gets the product that cheap. The supplier wouldn't be able to afford it. But most suppliers will take a loss on a product for Wal-Mart for Black Friday just for continued goodwill. It's also generally not a huge amount of whatever-it-is.