> ## Documentation Index > Fetch the complete documentation index at: https://docs.beinfi.com/llms.txt > Use this file to discover all available pages before exploring further. # Sell a product > Enrolls the customer in the product and opens an invoice for it, already `open` and ready to be paid. When the payment confirms, product delivery (email and download link) happens on its own. The customer is identified by `customer.externalId`: calling again with the same `externalId` reuses the customer and enrollment instead of creating duplicates. The amount comes from the product's published price unless you send `amount`. ## OpenAPI ````yaml /api-reference/openapi.en.json post /billing/products/{productID}/invoices openapi: 3.1.0 info: title: Infi API version: 0.0.1 description: > The Infi REST API: catalog, customers, usage metering, subscriptions, invoices, payments and checkout. **Authentication.** Send your secret key as `Authorization: Bearer sk_test_…` (sandbox) or `sk_live_…` (production). The key prefix selects the environment. Checkout routes (`/pay/*`) and `/public/*` take no key: they run in the payer's browser. **Idempotency.** Every request that changes data requires an `Idempotency-Key` header. A retry with the same key returns the original response instead of running again. **Amounts.** Money and quantities travel as decimal strings (`"150.00"`, `"0.0001"`), never as floating-point numbers. **Errors.** The body carries `error_code` (stable, branch on it), `message` (for humans) and `tracer_id` (send it to support). Rate-limit and idempotency errors are nested under `error`, with `code` and `request_id`. servers: - url: https://api-sandbox.beinfi.com description: Sandbox - url: https://api.beinfi.com description: Production security: - bearerAuth: [] tags: - name: Catalog - name: Customers - name: Metering - name: Subscriptions - name: Invoices - name: Payments - name: Checkout - name: Coupons - name: Billing - name: Account - name: Webhooks - name: Test account - name: Rail - name: Storefronts paths: /billing/products/{productID}/invoices: parameters: - $ref: '#/components/parameters/ProductID' post: tags: - Invoices summary: Sell a product description: > Enrolls the customer in the product and opens an invoice for it, already `open` and ready to be paid. When the payment confirms, product delivery (email and download link) happens on its own. The customer is identified by `customer.externalId`: calling again with the same `externalId` reuses the customer and enrollment instead of creating duplicates. The amount comes from the product's published price unless you send `amount`. operationId: createProductInvoice requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/CreateProductInvoiceRequest' responses: '201': content: application/json: schema: $ref: '#/components/schemas/Invoice' description: Created. '401': $ref: '#/components/responses/Unauthorized' '404': $ref: '#/components/responses/NotFound' '422': $ref: '#/components/responses/ValidationFailed' components: parameters: ProductID: name: productID in: path required: true schema: type: string format: uuid description: Product ID. schemas: CreateProductInvoiceRequest: type: object required: - customer properties: customer: type: object required: - externalId properties: externalId: type: string email: type: - string - 'null' name: type: - string - 'null' taxId: type: - string - 'null' description: >- The buyer. `externalId` is required and identifies the customer stably (their id in your system, or their email): the same key reuses the existing customer. `email`, `name` and `taxId` (CPF or CNPJ) are optional. amount: type: - string - 'null' description: >- Amount to charge, as a decimal string. Defaults to the product's published price. description: type: - string - 'null' description: Line description on the invoice. Defaults to the product name. currency: type: string description: Currency, as an ISO 4217 code. Defaults to `BRL`. dueDate: type: - string - 'null' format: date-time description: Due date. send: type: boolean description: '`true` emails the invoice to the buyer as soon as it is created.' successUrl: type: - string - 'null' description: Where checkout sends the buyer after payment. cancelUrl: type: - string - 'null' description: Where checkout sends the buyer if they back out. Invoice: type: object properties: id: type: string format: uuid customerId: type: - string - 'null' format: uuid description: >- The customer's product enrollment id. Set on product, usage and subscription invoices. payerId: type: - string - 'null' format: uuid description: The paying customer's id. Set on one-off invoices. subscriptionId: type: - string - 'null' format: uuid description: The subscription that generated the invoice invoiceNumber: type: - string - 'null' description: Sequential invoice number status: type: string enum: - draft - open - paid - void - uncollectible description: >- `draft` (being built), `open` (issued, awaiting payment), `paid`, `void` (canceled) or `uncollectible` (written off). currency: type: string description: Currency, as an ISO 4217 code (`BRL`). subtotal: type: string description: Sum of line items before tax and discounts. tax: type: string description: Tax on the invoice. total: type: string description: The invoice's final amount. amountDue: type: string description: How much is still owed. periodStart: type: - string - 'null' format: date-time description: Start of the billed period periodEnd: type: - string - 'null' format: date-time description: End of the billed period dueDate: type: - string - 'null' format: date-time description: Due date. finalizedAt: type: - string - 'null' format: date-time description: When the invoice was issued. paidAt: type: - string - 'null' format: date-time description: When the invoice was paid. successUrl: type: - string - 'null' description: Where checkout sends the buyer after payment. cancelUrl: type: - string - 'null' description: Where checkout sends the buyer if they back out. createdAt: type: string format: date-time lineItems: type: array items: $ref: '#/components/schemas/InvoiceLineItem' description: >- What a customer owes: line items, totals and status. Status: `draft` → `open` → `paid`, or `void` / `uncollectible`. InvoiceLineItem: type: object properties: id: type: string format: uuid type: type: string enum: - usage - recurring - proration - credit - adjustment - commitment - manual description: >- `usage` (metered usage), `recurring` (recurring fee), `proration` (pro-rata adjustment for a mid-period change), `credit`, `adjustment` (usage that arrived after its period was already invoiced), `commitment` (top-up to the minimum commitment) or `manual` (free-form line). meterId: type: - string - 'null' format: uuid description: The meter the usage came from priceId: type: - string - 'null' format: uuid description: The price applied to this line. description: type: string quantity: type: string description: Quantity billed unitPrice: type: - string - 'null' description: Price per unit amount: type: string description: Line total proration: type: boolean description: '`true` when the line is a pro-rated amount.' description: One line on an invoice. Error: type: object required: - message - error_code - tracer_id properties: message: type: string description: Human-readable sentence. error_code: $ref: '#/components/schemas/ErrorCode' description: Stable error code. Branch on it. tracer_id: type: string description: Request id. Include it when contacting support. description: >- The standard error body. It is flat (no `error` wrapper) and the trace id is `tracer_id`. NotFoundError: type: object required: - message - error_code - tracer_id - resource properties: message: type: string error_code: $ref: '#/components/schemas/ErrorCode' tracer_id: type: string resource: type: string ValidationError: type: object required: - message - error_code - tracer_id - errors properties: message: type: string error_code: $ref: '#/components/schemas/ErrorCode' tracer_id: type: string errors: type: array items: type: object required: - field - value - constraint - description properties: field: type: string value: type: string constraint: type: string description: type: string description: >- Per-field detail (`field`, `description`). Always present on a `422`; may be empty. description: >- Body of a `422`. Same shape as `Error`, plus `errors[]` with per-field detail. That is where the sentence telling you what to fix lives (for example, "product has no published version"). ErrorCode: type: string examples: - internal_error - validation_failed - bad_request - unauthorized - forbidden - not_found - conflict - rate_limited description: >- Stable error code. The list is open: besides the generic codes, each area defines its own (`coupon_expired`, `insufficient_balance`, `version_not_draft`, `idempotency_key_reused`…). Handle a code you do not recognize by its HTTP status and show `message`; do not fail parsing the response. responses: Unauthorized: content: application/json: schema: $ref: '#/components/schemas/Error' description: Missing or invalid API key. NotFound: content: application/json: schema: $ref: '#/components/schemas/NotFoundError' description: Resource not found. ValidationFailed: content: application/json: schema: $ref: '#/components/schemas/ValidationError' description: One or more fields are invalid. Per-field detail is in `errors[]`. securitySchemes: bearerAuth: type: http scheme: bearer description: >- Your account's secret key: `Authorization: Bearer sk_test_…` or `sk_live_…`. Server-side only. ````