# invoice Specification

## Purpose
Invoice requests raised by buyers and their issuance by shops.
## Requirements
### Requirement: Invoice request
Customers SHALL request at most one open invoice (发票) per order, with title, kind (`personal | company`), and tax_no required for company invoices. Amount equals the order total in the order currency.

#### Scenario: company invoice requires tax number
- **WHEN** requesting a company invoice without tax_no
- **THEN** the API returns 400

#### Scenario: duplicate rejected
- **WHEN** an order already has a requested or issued invoice
- **THEN** a second request returns 409

### Requirement: Invoice issuance
Merchants SHALL issue requested invoices of their own shop's orders; issuing sets invoice_no, issued_at and status `issued`.

#### Scenario: issue flow
- **WHEN** the shop issues a requested invoice
- **THEN** the customer sees status `issued` with an invoice number