If you've shopped at a physical store, you know it can be frustrating to pick out a product that's on display, and then have to wait while a salesperson searches for it among the stacks of boxes in the stockroom. E-commerce warehouses strive to avoid that logjam — and speed up deliveries — by having accurate inventory information.

Whenever inventory is low, they automatically replenish the inventory by sending an order. The systems have centralized optimization engines, which decide how to dispatch inventory to different fulfilment centers.

Positioning products in the warehouse according to demand, rather than category, is another trick that speeds things up. Amazon warehouses, for instance, don't have toy sections, or food sections or even book sections. That, coupled with making the most frequently ordered stuff the most accessible, enables warehouse workers to find products in a hurry and get them onto trucks.

E-commerce retailers also use another technique called cross-docking, originally pioneered by Walmart, for getting goods out of big trucks and directly into smaller vans for delivery. The idea of cross-docking is to reduce the need for warehousing as much as possible. Basically, outgoing trucks that pick up from incoming trucks, without having to deal with shelving, and sorting and warehousing operations.

Next-day delivery works pretty well these days, but with consumers growing accustomed to it, online retailers increasingly are offering same-day delivery for some items. A 2021 forecast by ResearchAndMarkets.com predicted that the same-day delivery market would grow at a compound rate of more than 20 percent between 2021 and 2025.