A recent patent filing unearthed by 3DPrint outlines a method in which Amazon could 3D print a customer's order on-demand. The manufacturing process wouldn't take place in some obscure warehouse but rather, in a company delivery vehicle as it's heading your way to drop off an order.
These mobile manufacturing hubs could be loaded with all sorts of equipment to both additively and subtractively manufacture an order. And with a large mobile fleet, Amazon could lessen the amount of warehouse space it needs that would normally be occupied by 3D printing equipment, products, robots to pick orders and physical employees to oversee the process.
Keep in mind, however, that not all patent filing ideas end up in the real world. More often than not, companies will file patents like this to help cover all of their bases legally, open itself up to options down the road or even prevent the competition from capitalizing on an idea originally conceived in-house.
With this particular patent, it could get rather expensive to deploy and operate a fleet of mobile printing hubs - especially if the demand for 3D printed items isn't strong enough.