Next-day and same-day delivery are achieved by deciding where inventory sits, not by moving parcels faster. The network design behind that decision explains most of what customers experience.
Distance to the customer is fixed before the order
Once an order is placed, the delivery time is largely determined by where the item already is, because transport speed has limited room to vary.
A parcel sent from a distant warehouse must cross the network overnight, while one held locally only needs a final delivery run.
Promising a short delivery window therefore requires the stock to be positioned in advance, which is a forecasting problem rather than a logistics one.
Holding stock in many places multiplies inventory
Serving a country from one location requires far less total stock than serving it from twenty, because demand variation averages out across a larger pool.
Splitting inventory means each site must hold buffer stock for its own uncertainty, and the total held rises even though sales do not.
Fast delivery is therefore purchased with working capital, and the cost falls on items that sell slowly and unpredictably.
Assortment is split by velocity
Retailers respond by holding fast-moving items in many locations and slow-moving items in a few central ones.
This is why delivery estimates differ between products on the same site, with popular goods offered next day and specialist items quoted several days out.
The split is recalculated continuously as demand patterns shift, which is why an item's available speed can change between visits.
The final leg dominates cost
Moving a full trailer between hubs is inexpensive per parcel, while delivering individual items to separate addresses is not.
Density is what makes that leg affordable, since a driver covering many drops in one street achieves a far lower cost per parcel than one covering a rural round.
Collection points and lockers exist to recreate density artificially, concentrating many deliveries at a single stop.
Returns run the network backwards
Returned items arrive individually from many addresses and must be inspected, sorted and either restocked or disposed of.
That process is slower and more labour-intensive than outbound fulfilment, and it competes for the same warehouse space and staff.
Categories with high return rates therefore carry a structural cost that shapes where they are stocked and how quickly they can be offered.