# Invoice "Set" Pricing — Design & Front-/Back-end Contract

Customer requirement: items declared as a **set** in `[dbo].[mfr__items]`
(`[Type] = 'set'`) should normally be shown as a single **set price** on the
invoice instead of being broken up into their member items and summed.

Three display modes (switchable in the invoice editor):

| Mode | Set line | Member items | Use as |
|---|---|---|---|
| **SetPrice** (default) | shown **with price** | shown **without price** | the new default |
| **ItemPrices** | shown as a heading **without price** | shown **with price** | the previous behaviour |
| **SetOnly** | shown **with price** | **removed** | compact |

> **Totals are unaffected.** The invoice total is taken from the registration
> balance (`InvoiceBalance` / `InvoiceBalance_net`), not by summing the rendered
> lines, so switching modes is purely presentational. The set price always
> equals the sum of its members (computed as a fallback when the set header
> carries no own price).

## Back-end (implemented + unit-tested)

- `Fuchs/code/InvoiceSetPricing.cs` — the authoritative transformation:
  `SetDisplayMode` + `Build(items, mode)` → ordered `InvoiceSetLine`s, each with
  `ShowPrice` and `IsSetHeader`. `ModeFromInvoiceOptions(...)` reads the mode
  from the invoice options. Fully covered by `Fuchs.Tests/InvoiceSetPricingTests.cs`.
- `FuchsPdf.ApplyInvoice` renders through `InvoiceSetPricing.Build`: lines with
  `ShowPrice == false` render **blank** price/total cells (not `0,00 €`), and the
  set header line is rendered emphasised. Invoices without sets pass through
  unchanged.

## Front-end contract (to wire in the invoice editor)

The editor already knows the set structure (from `fds__prepInvoice`) and assembles
the `invc` JSON. To drive the modes it must, when sending the invoice:

1. **Mode** — add a token to `inv.InvoiceOptions` (CSV, alongside `§13b`):
   - `setmode:setprice` (default — may be omitted), `setmode:itemprices`, or `setmode:setonly`.
   A 3-way switch in the editor sets this token.

2. **Item flags** — in each service request's `items[]`:
   - the **set header** item: `type: "set"`, `id: "<setId>"`, and `total_net` =
     the set price (or `0` to let the back-end sum the members);
   - each **member** item: `setId: "<setId>"` (matching the header's id).
   - standalone items need no extra fields.

That is the entire contract — the back-end does the rest. The editor's running
**total stays the member sum in every mode**, matching the registration balance.

### Why the switch lives in the editor
Set grouping is only known where the request/item tree is rendered (front-end).
The back-end intentionally stays the single, tested authority for *how* a chosen
mode maps to printed lines, so the editor only needs to pick the mode and tag the
items — it does not re-implement the pricing rules.

## Persistence note
Draft/preview PDFs render straight from the posted `invc` JSON, so the contract
works end-to-end for previews immediately. For a **finalised** invoice to re-render
in a chosen mode later, the per-item `type`/`setId` flags (and the `setmode`
option) must be persisted with the stored invoice items
(`fds__createInvoice_Details` / `fds__invoice_items`). `setmode` already persists
via `InvoiceOptions`; persisting the per-item set tags is a small SSDT + create-proc
change to make once the editor wiring is confirmed.