Yes, FIZ has an API.

A REST API to issue certified invoices, create customers and items, and sync with the Portuguese tax authority (AT) — from your own software, store or platform. Authenticate with one key and issue your first invoice today.

Get an API key Talk to the team

Issue a certified invoice in a few lines

Authenticate with an API key in the x-api-key header and POST to create and issue the document. The AT is notified automatically and you get back the PDF with ATCUD and QR code.

Read the docs → Generate my key →

# Issue an invoice-receipt
curl -X POST https://api.fiz.co/invoices \
  -H "x-api-key: $FIZ_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "type": "INVOICE_RECEIPT",
    "customerId": "cus_123",
    "items": [{ "itemId": "itm_42", "quantity": 10 }]
  }'

What teams build with it

The most common builds from teams already integrating FIZ.

Online store → automatic invoice

Connect your store (Shopify, WooCommerce and similar) and issue the certified invoice the moment a sale completes — with no manual work.

Invoice when the customer pays

After your gateway (Stripe, ifthenpay, easypay…) confirms the payment, call FIZ to issue the document and email it to the customer.

Embed FIZ in your SaaS

Running a platform, marketplace or management software? Let your own customers invoice straight from your product, with FIZ handling tax compliance.

Migrate from another invoicing tool

On Moloni, InvoiceXpress, Sage or Vendus just for the API? You get what you need on FIZ — with AT sync already built in.

What you can do

Everything FIZ does in the app, available programmatically.

Invoices & receipts

Create drafts and issue Invoice, Invoice-Receipt, Simplified Invoice, Debit Note and Credit Note — with discounts, payment methods and withholding tax.

Customers & items

Create, update and search customers with validated NIF, and manage your product and service catalogue with VAT rates and exemptions by territory.

PDF, ATCUD & QR code

Download each document’s PDF with your own templates, ATCUD and QR code — ready to send to the customer.

Tax authority sync

Every issued document is automatically reported to the Portuguese tax authority. Track the sync status on every call.

For Claude and ChatGPT

Invoicing in plain language

We published an open-source skill that connects your AI assistant — Claude or ChatGPT — to the FIZ API. Just say “bill João 10 consulting hours” and the assistant handles the rest, with Portuguese tax knowledge built in: VAT rates, exemption codes, NIF validation and a required confirmation before the irreversible issue step.

View on GitHub MIT licensed · installs in seconds

Platforms & integrators

Building on top of FIZ?

If you’re integrating FIZ into a platform or need something that isn’t there yet — several companies under one account, per-client keys, webhooks, a test environment or a specific endpoint — talk to us. We’ll look at your use case and help get the integration working.

Talk to support →

Ready to integrate?

Generate your API key in your account settings and issue your first invoice from your own code.

Get an API key