Amazon plans to ship your packages before you even buy them

It's common in supply chain management to optimize the ordering and placement of items before they are "consumed". If you store everything in one central location, there's an increased cost to quickly move it to the final destination. If you distribute some items to regional hubs, there's a (wasted) cost to doing that too, especially if you ship more or less than is actually needed. They try to estimate what items go where based on their historical data: what people buy where and how often. They could speculatively do this with "people who buy this will often buy that" data too.

In this case, they look at the cost of returning an excess item from a regional hub to an upstream distribution center and try to recoup the cost by selling to someone near it's current location at a discount that is less than the cost of moving it around further.