We came to Thornton, Colorado, today to visit an Amazon Fulfillment Center. This is where your order is shipped from. It was mind boggling. Nothing is put on shelves. All the inventory is put into thousands of pods which are portable shelves which are shuttled around by robotic drive units. Each pod has dozens (hundreds?) of different products. They are moved around and appear at just the right time at the picking station. The Fulfillment Center is immense. Receiving, sorting, picking, packing and shipping all in one place. If you ever get the chance for a tour, take it. Here is a good video.