api/documentation/economic/invoice-discount-format-drift12.md

# Invoice Discount Format — DRIFT 12 (TRU-73)

## What changed

The e-conomic draft invoice now applies the **customer-level discount
percentage at the line level** on every line item, so the discount is
clearly visible on each service line on the customer's invoice.

Before this fix, a customer with a global e-conomic discount (e.g. the
`kd` customer `35131752` with a 15% discount) would receive an invoice
where the discount was only reflected via an aggregate `TotDiscount`
line — and crucially, e-conomic's draft invoice **line** API requires
`discountPercentage` on each line, so the aggregate line was being
ignored entirely. The customer was getting invoiced at full price with
no visible discount at all.

## Invoice layout — before vs after (for Jimmy)

The example below uses customer `35131752` ("kd") with a 15% global
e-conomic discount, ordering one wash line at 100.00 DKK.

### Before the fix (DRIFT 12 — discount silently dropped)

```
─────────────────────────────────────────
  Vask                                     1 ×   100,00 DKK     100,00
─────────────────────────────────────────
                                       Subtotal                100,00 DKK
                                          Rabat (15%)            0,00 DKK   ← never applied
                                       Total                   100,00 DKK
─────────────────────────────────────────
```

The `Rabat` line was never actually created on the e-conomic side
because the customer has a per-line discount configured, not an
aggregate one. The customer saw 100,00 DKK with no discount displayed.

### After the fix (TRU-73)

```
─────────────────────────────────────────
  Vask (15% rabat)                        1 ×   100,00 DKK     100,00
                                                          Rabat:  -15,00 DKK (15%)
─────────────────────────────────────────
                                       Subtotal                100,00 DKK
                                          Rabat                 15,00 DKK
                                          Total                 85,00 DKK
─────────────────────────────────────────
```

The 15% discount now appears on the wash line itself (via the
`discountPercentage` field that e-conomic renders on each line), and
the subtotal correctly reflects the 85,00 DKK total the customer owes.

## How the fix works

1. The customer discount percentage is resolved from the cached
   `economicCustomers` record (via Redis when available, otherwise
   through the live e-conomic API) and threaded through
   `economic_invoice_draft::addOrderItemLines()` /
   `addOrderItemLine()`.
2. On each line, the customer discount is combined with the per-item
   discount using `max(per_item, customer)` so the larger discount
   always wins — the system never accidentally double-discounts a
   line that already has a per-item price reduction.
3. The aggregate `TotDiscount` line is suppressed when the customer
   has a per-line discount, since e-conomic's draft line API requires
   `discountPercentage` to be on the line itself.
4. The customer discount is clamped to 0..100 to guard against bad
   data from the e-conomic API.

## Code paths

- `services/nginx/app/modules/economic/helpers/economic_invoice_draft.php`
  — `addOrderItemLines()` and `addOrderItemLine()` now accept a
  `customer_discount_percentage` argument and combine it with the
  per-item discount at the line level.
- `services/nginx/app/modules/economic/customers/economicCustomers.php`
  — logs swallowed missing-currency-price errors so silently-missing
  discounts become visible in the application log.
- `services/nginx/app/modules/economic/endpoints/invoices/draft/economic_invoices_draft_endpoint.php`
  — forwards the customer discount percentage to the draft builder.
- `services/nginx/app/objects/collected_order_invoices_o.php`
  — resolves the customer discount via the Redis cache + e-conomic
  customer index and passes it to the draft builder.

## Tests

- `services/nginx/app/tests/Unit/Invoicing/EconomicInvoiceDraftCustomerDiscountTest.php`
  — new tests covering the customer 35131752 case (15% global discount,
  applied at line level) plus edge cases: per-item + customer discount
  combined, clamping to 0..100, zero-discount baseline.
- `services/nginx/app/tests/Unit/Invoicing/EconomicInvoiceDraftDiscountLineModeWiringTest.php`
  — updated to account for the new parameter and the customer-discount
  guard on the aggregate `TotDiscount` line.
- `services/nginx/app/tests/Unit/Invoicing/CollectedInvoiceEconomicBatchTransferWiringTest.php`
  — updated to thread the new parameter through the batch transfer
  pipeline.