A reservation answers the commonest question before it's asked. Someone who can see online that an item is in stock, and have it put aside, doesn't ring — and comes in anyway.
Deliveries are logistics and should be planned like it. Addresses, time windows, who's driving and what goes with them: as a round in the morning rather than a pile of notes in the afternoon.
The notification is precisely where orders stall. When something ordered arrives and nobody says so, it sits in a drawer for a week and the customer buys it somewhere else.