# Domain: Orders, Checkout, Fulfillment, Freight

## Lifecycle

```
cart → checkout (one tx) → order per shop          shipment flow
─────────────────────────                          ─────────────────────────
pending_payment ──pay──► paid ──first shipment──► fulfilling ──all shipped──► shipped
      │                                                    │
      └──cancel──► cancelled                    delivered (all shipments) ──► completed
```

Every arrow is a guarded `UPDATE … WHERE status = …`; stale actions → 409.

## Checkout invariants (`modules/order/service.rs`)

One transaction per checkout: lock SKUs (`FOR UPDATE` in id order) → resolve
flash-sale activity pricing → per-shop coupon discount (server-computed,
client sends only coupon ids) → per-shop freight fee → insert order + items +
stock decrements → commit → clear cart. Orders are split per shop; each order
carries `total_minor = subtotal − discount + shipping_fee_minor`.

Snapshots make history immutable: order items keep `unit_price_minor`,
`product_name`, `flash_sale_item_id`, and the freight
`freight_template_id`/`freight_pricing_method` resolved at checkout. Later
template edits never touch past orders.

## Freight (`modules/freight/`)

Templates are shop-scoped (`own_shop`): by_piece or by_weight (grams on
`skus.weight_grams`), first-unit + additional-unit fees with part-unit
round-up, `always_free`, `free_threshold_minor`, and region rules overriding
default fees by case-insensitive match against address region/city/country.
Resolution per line: product template → shop default → zero fee. Fees merge
per template group within a shop order and convert from the group's SKU
currency to the order currency.

`POST /orders/shipping-quote` is the read-only quote (normal prices only);
checkout recomputes authoritatively in the order transaction. Client-supplied
fees are never read.

Shipping companies are a dictionary (`shipping_companies`); shipments record
`shipping_company_code` validated against it.

## Key files

See `docs/code_index/order.md`.