It doesn’t happen a lot, especially if you’re an Amazon Prime member, but occasionally if you order an item from an Amazon third-party seller you might end up with an item shipped in a Target box, or containing a shipping receipt from Walmart. It’s not illegal or a scam, but it probably means you paid more than you should have.

CNBC’s Courtney Reagan writes about an experiment her team tried, ordering a bunch of identical items from Amazon, Walmart.com, and Target.com, shipped to various addresses around the country. They didn’t use an Amazon Prime membership, as Prime items are generally purchased directly from Amazon or fulfilled through Amazon warehouses.

One item they ordered a bunch of were bookshelf sets. All four of the sets ordered from third-party Amazon sellers arrived in packaging still covered in Walmart labeling.

Another item purchased from Amazon third-party sellers was Tide detergent. This time, CNBC reporters received detergent that had been purchased and resold from Walmart and Target. They know this because there were actual shipping orders from those sites in their respective boxes.

It’s a practice known as “retail arbitrage,” which is just a fancy way of saying “buying stuff from Retailer A to resell on your own.”

It’s nothing new. My first job ever was at a mom-and-pop grocery store, and I can recall a number of times when the owner bought things like cases of soda or bottled water at another store just to keep shelves stocked while waiting for a delivery from his distributor.

There’s also an ongoing international legal battle involving Pirate Joe’s, a small Canadian retailer that stocks his store with items purchased at Trader Joe’s stores across the border.

As CNBC points out, Amazon doesn’t allow third-party sellers to buy items directly from a competing online retailer and have it shipped to the customer. That doesn’t mean it doesn’t happen, but the instances in this story appear to just be cases where Amazon sellers are buying for less from Walmart or Target then re-shipping.

Aside from clearing up the mystery of why the shelves you order from Amazon might come in a box with Walmart tape or stocking label, the story is a reminder that you should never assume Amazon has a lower price than websites for bricks-and-mortar retailers.

For example, the shelves bought from Amazon sellers averaged $15.22 more than the $68 Walmart.com price, while the Tide from Target and Walmart was, on average $10.26 less expensive than the price charged by the Amazon sellers.

So it just goes to show that doing a quick price check on a few e-tail sites is always a good idea.

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