Customers are not choosing between shops. They are choosing between a Thursday evening slot and no slot, and once it is full the order cannot happen at all.
The scarce thing is not stock. It is how many baskets a van can carry on Thursday evening.
Two customers taking the last Thursday slot at once is a delivery you cannot make. Slot allocation is serialised so the second one is told it is gone rather than accepted.
Perishable availability changes through the day as the shop sells and receives. Figures are refreshed on a short cycle rather than cached until tomorrow.
Discovering at checkout that you do not deliver to an address wastes the customer's twenty minutes. The zone check happens at the front rather than the end.
When an item is out, the picked alternative and the price difference both have to be reflected. That is an order amendment, not a note.
Baskets are built at lunchtime and in the evening. Capacity follows that rhythm rather than a flat daily figure.
An oversold delivery slot is worse than an out-of-stock product. The customer has arranged their evening around it, and there is no substitute for a van that is already full.
The measurement is two customers reaching for the same remaining slot simultaneously, because that is the moment the system either holds or oversells.
The constrained resource is delivery capacity rather than inventory, and everything follows from that.
| Setting | What we do | Why |
|---|---|---|
| Slot allocation | Serialised so capacity cannot be exceeded | Two shoppers accepting the last slot simultaneously produces a delivery that physically cannot be made, and neither customer did anything wrong. |
| Perishable availability | Refreshed on a short cycle with affected pages purged | Fresh stock changes through the trading day, so a figure cached overnight sells things the shop no longer has by mid-morning. |
| Delivery zone validation | Performed before the basket is built | Finding out at checkout that the address is outside the area wastes twenty minutes of the customer's time and loses the order entirely. |
| Substitution handling | Recorded as an amendment with the price difference | A picked alternative changes what was bought and what is owed, so treating it as a note leaves the order and the charge disagreeing. |
| Daily peaks | Capacity planned for lunchtime and evening ordering | Baskets are built in two windows each day, and a flat average describes neither of them. |
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