# Issue the GST tax invoice for an order

`POST /api/v1/india-gst/invoices`

Issues the GST tax invoice for an order, computing the lines, the rates and the CGST/SGST/IGST split from the order itself and drawing a number from a GAPLESS per-financial-year sequence. IT MINTS A LEGAL DOCUMENT. Unlike every other create on this API it cannot be undone by removing the row, because the series has already moved. 201 EVEN ON A REPEAT, and the body is the invoice NOW IN FORCE. An order that already has a standing invoice gets that one back without consuming a second number, so a retrying pipeline cannot punch holes in the sequence. Compare the id you get back if you need to know which happened; branching the status on it would tell a key holding only the write grant whether an invoice exists, which is a read. THE STORE MUST HAVE GST ENABLED. A store with no configuration, or one with enabled false, is refused 409 rather than issuing an untaxed document. EVERYTHING THE DOCUMENT ASSERTS ABOUT ITSELF IS REFUSED BY NAME: the number, the financial year, the five amounts, the lines and issued_by are computed or stamped here. Only order_id and place_of_supply are yours to send. issued_by is stamped and NOT published back: the column holds a merchant staff user id on a dashboard-issued invoice, and a scope over tax documents is not a grant over the staff directory.

## Headers

- `Idempotency-Key` (string, required) — A unique key per logical write. Replaying a request with the same key returns the first response byte for byte instead of applying the write twice.

## Request body

Required.

- `order_id` (string, required) — The order to invoice. It must belong to this store, and its own line items are what the invoice is computed from.
- `place_of_supply` (string) — The RECIPIENT's two-digit GST state code, 01 to 38. Equal to the supplier's state means an intra-state supply and a CGST/SGST split; anything else means IGST. Omit it for the supplier's own state. A code outside the set is a 400 rather than a guess, because guessing wrong taxes the supply under the wrong heads.

## Responses

- `201` — Created
- `400` — The request was refused before any state changed. `code` is one of:

`invalid_body`, a write body this route will not take. `reason` partitions it
and `field` names the key when one key is at fault.
`invalid_query`, a query parameter, including limit and after.
`invalid_text`, a NUL byte or bytes that are not valid UTF-8 anywhere in the
path, the query or the body. Strip control characters before sending.
`idempotency_key_required`, a write sent without the Idempotency-Key header.
`invalid_request`, an Idempotency-Key longer than 255 bytes.

Routes add their own codes for rules only they know. Switch with a default arm.
- `401` — No credential, or one this API does not accept. `code` is always
`unauthorized`.

THE BODY IS DELIBERATELY UNINFORMATIVE. An expired key, a revoked key, a
publishable key, a key belonging to another merchant and a key that never
existed are all refused with the same bytes, so this response cannot be used to
probe which keys exist. Check the key's state in the dashboard rather than
inferring it here.

Send the key as `Authorization: Bearer <key>` or as `X-API-Key: <key>`. It is
never accepted in a query string.
- `403` — The key lacks the write_india_gst scope. `code` is `insufficient_scope` and the message names the scope to ask the merchant for.
- `409` — The request is well formed and the store's current state refuses it.

EVERY WRITE CAN ANSWER TWO OF THESE, whatever it does:
`idempotency_key_reused`, this Idempotency-Key already served a different
method, path or body. The key namespace is per store and not per endpoint, so a
key built from a business id collides across routes; use a fresh key per
logical write.
`idempotency_key_in_progress`, an identical request is still running. Retry
after a short delay; exactly one of the racing calls will have applied.

`not_configured` means the merchant has not set something up yet, with
`resource` naming what. The remaining codes are the per-route state rules named
on the operation.
- `413` — The request body exceeded the cap named in the message. `code` is
`payload_too_large`. Nothing was read and nothing was recorded, so the same
Idempotency-Key may be reused once the body is smaller.
- `415` — The body was not declared as `application/json`. `code` is
`unsupported_media_type`.

This API reads ONE media type. The `+json` structured suffixes are refused
too, because they name semantics (merge-patch in particular) this API does
not implement, and reading such a body as plain JSON would be a silent
misreading. A write that carries NO body needs no Content-Type at all.
- `422` — An id INSIDE the request resolves to nothing on this store. `code` is
`unprocessable_reference`, `resource` names the kind of thing that did not
resolve and `field` names where it arrived, so the fix is mechanical.

Distinct from 404, which is about the url, and from 400, which is about the
bytes. The message stays generic about WHY the id did not resolve: an id owned
by another merchant must read identically to one that never existed.
- `429` — Too many requests. `code` is `rate_limited`.

Two limits apply independently: one on the credential and the route family,
one on the client address. The headers describe whichever has less left, so
honouring Retry-After always clears the window that bound.
- `default` — Any status this operation does not list, in the same envelope.

A 5xx means the request may or may not have applied. Retry it with the SAME
Idempotency-Key: that is the only way to find out without risking a duplicate,
and it is what the key is for.

A few 4xx conditions arrive here rather than as a listed status because they
depend on the merchant's plan or on a module being wired: 402 when a quota or a
plan limit is reached, and 503 when a capability the route needs is not
configured on this deployment. Both carry a `code` naming which.

## Example

```bash
curl --request POST \
  --url 'https://api.mercemur.com/api/v1/india-gst/invoices' \
  --header 'Authorization: Bearer <token>'
```