Invoices and receipts, delivered with a paper trail

Attach the PDF, pass the order details as template variables, and send exactly once — even when your billing job retries. Every message keeps an id, an event history and its attachment list for later lookups.

Start sending free Read quickstart

Up to 20 attachments per email
Invoice number as idempotency key
reply_to your finance inbox
Event history per message

email api request

ready

send-invoice.tsattachments

await fetch("https://sendix.dev/api/v1/emails", {
  method: "POST",
  headers: {
    Authorization: "Bearer " + process.env.SENDIX_API_KEY,
    "Content-Type": "application/json",
    "Idempotency-Key": "invoice-" + invoice.number,
  },
  body: JSON.stringify({
    from: "Acme Billing <[email protected]>",
    to: [customer.email],
    reply_to: "[email protected]",
    template_id: INVOICE_TEMPLATE_ID,
    template_data: { number: invoice.number, total: invoice.total },
    attachments: [{
      filename: "invoice-" + invoice.number + ".pdf",
      content: pdf.toString("base64"),
      contentType: "application/pdf",
    }],
  }),
});

attachmentsreply_toIdempotency-Key

Short answer

How do I email invoices and receipts with Sendix?

POST /v1/emails with the PDF in attachments (base64 content, filename and contentType; up to 20 attachments per message), the invoice fields in template_data and an Idempotency-Key such as the invoice number. Store the returned email id with the invoice so finance or support can check GET /v1/emails/{id}/events and GET /v1/emails/{id}/attachments later.

How it works

Four steps from app event to delivered email.

What your code does, what Sendix does, and where to look when something goes wrong.

01
your app
Generate the document
Render the PDF in your billing system and base64-encode it for the attachments array.
02
Idempotency-Key
Send once per document
Use the invoice or order number as the Idempotency-Key, so retries never email a customer twice.
03
email id
Keep the email id
Save the id returned with 202 Accepted next to the invoice record for support and audit lookups.
04
webhooks
Handle failures
On email.bounced or email.failed, alert your billing team or fall back to another contact on the account.

Best practices

Details that keep this mail reliable.

Habits that protect deliverability and your users, whichever provider you use.

Put key facts in the body

Amount, due date and invoice number in the email itself make it useful even before the PDF is opened.

Route replies to finance

Set reply_to to a monitored inbox, or receive replies through Sendix inbound and route them to your helpdesk.

Use a billing sender

A consistent billing@ address helps customers recognise and filter financial mail.

Keep attachments lean

Compress PDFs and avoid unnecessary files; smaller messages are faster to process and less likely to be rejected.

Send over SMTP if your platform does

E-commerce and billing tools that only speak SMTP can use smtp.sendix.dev:587; MIME attachments pass through.

Keep billing separate from marketing

Don’t add promotions to invoices; it blurs the transactional nature of the message.

Webhook events

The events worth subscribing to.

Sendix signs each webhook with X-Sendix-Signature and keeps the same events on the message at GET /v1/emails/{id}/events.

Event	What to do with it
email.sent	What to do with itQueued for delivery.
email.delivered	What to do with itAccepted by the customer’s mail server.
email.bounced	What to do with itPermanent failure — update the billing contact.
email.failed	What to do with itSendix could not deliver the message.
email.complained	What to do with itMarked as spam by the recipient.

FAQ

Frequently asked questions

Short answers to the questions teams ask while wiring this up.

Can I attach PDF invoices?

Yes. The API accepts up to 20 attachments per message, each with a filename, base64 content and an optional contentType. Over SMTP, standard MIME attachments are passed through.

How do I avoid emailing an invoice twice?

Send the invoice number as the Idempotency-Key. A repeated request with the same key returns the original email id instead of creating a new message.

Where do customer replies go?

To the reply_to address you set. If you enable receiving on a domain, replies can also arrive in Sendix as parsed messages and trigger an email.received webhook.