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March 26, 2026

How to Automate Invoice PDF Generation with a REST API

invoicesautomationREST APItutorial

Still creating invoices manually in Excel? Copying data between spreadsheets and praying you didn't mistype an amount? There's a better way.

With a PDF generation API, you can turn invoice data into professionally formatted PDFs — and email them to customers — with a single API call. This guide walks you through the implementation with real code examples.

Why Automate with an API?

The Manual Process Problem

  • Slow: Create in Excel → export to PDF → attach to email → send
  • Error-prone: Manual data entry leads to typos in amounts and addresses
  • Doesn't scale: 10 invoices per month is fine. 1,000 is a nightmare.

The API Approach

  • Seconds, not hours: Generate from database values instantly
  • Zero errors: Code handles calculations and formatting
  • Scales infinitely: Same code handles 10 or 10,000 invoices

Architecture Overview

Order data → HTML template → PDF Generation API → Email delivery

You need three things:

  1. An HTML template — the invoice layout
  2. Data — customer name, line items, amounts
  3. One API call — generates the PDF and sends the email

Step 1: Create an HTML Template

Create an invoice template using HTML and CSS. With FUNBREW PDF's template engine, use {{variable_name}} placeholders for dynamic values.

<div style="font-family: -apple-system, sans-serif; max-width: 800px; margin: 0 auto; padding: 40px;">
  <div style="display: flex; justify-content: space-between; margin-bottom: 40px;">
    <div>
      <h1 style="font-size: 28px; margin: 0;">Invoice</h1>
      <p style="color: #6b7280;">Invoice #: {{invoice_number}}</p>
      <p style="color: #6b7280;">Date: {{issue_date}}</p>
    </div>
    <div style="text-align: right;">
      <p style="font-weight: 700;">Your Company Inc.</p>
      <p style="color: #6b7280; font-size: 13px;">123 Business St, Suite 100</p>
    </div>
  </div>

  <div style="margin-bottom: 32px;">
    <p style="font-size: 18px; font-weight: 700;">Bill to: {{customer_name}}</p>
  </div>

  <table style="width: 100%; border-collapse: collapse; margin-bottom: 32px;">
    <thead>
      <tr style="background: #f8fafc;">
        <th style="text-align: left; padding: 12px; border-bottom: 2px solid #e5e7eb;">Item</th>
        <th style="text-align: right; padding: 12px; border-bottom: 2px solid #e5e7eb;">Qty</th>
        <th style="text-align: right; padding: 12px; border-bottom: 2px solid #e5e7eb;">Price</th>
        <th style="text-align: right; padding: 12px; border-bottom: 2px solid #e5e7eb;">Total</th>
      </tr>
    </thead>
    <tbody>{{line_items}}</tbody>
  </table>

  <div style="text-align: right; margin-bottom: 32px;">
    <p>Subtotal: ${{subtotal}}</p>
    <p>Tax: ${{tax}}</p>
    <p style="font-size: 20px; font-weight: 700;">Total: ${{total}}</p>
  </div>

  <div style="border-top: 1px solid #e5e7eb; padding-top: 16px; color: #9ca3af; font-size: 12px;">
    <p>Due date: {{due_date}}</p>
    <p>Payment: Wire transfer to Account #1234567</p>
  </div>
</div>

Register this template in the FUNBREW PDF dashboard.

Step 2: Generate an Invoice PDF via API

JavaScript (Node.js)

const response = await fetch("https://pdf.funbrew.cloud/api/pdf/generate-from-template", {
  method: "POST",
  headers: {
    "Authorization": "Bearer sk-your-api-key",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    template: "invoice",
    variables: {
      invoice_number: "INV-2026-0042",
      issue_date: "March 26, 2026",
      customer_name: "Acme Corp",
      line_items: buildLineItemsHtml(items),
      subtotal: "1,500.00",
      tax: "150.00",
      total: "1,650.00",
      due_date: "April 30, 2026",
    },
    email: {
      to: "billing@acme.com",
      subject: "Invoice INV-2026-0042 from Your Company",
    },
    options: { engine: "quality" },
  }),
});

const result = await response.json();
console.log("PDF URL:", result.data.download_url);

Python

import requests

response = requests.post(
    "https://pdf.funbrew.cloud/api/pdf/generate-from-template",
    headers={"Authorization": "Bearer sk-your-api-key"},
    json={
        "template": "invoice",
        "variables": {
            "invoice_number": "INV-2026-0042",
            "issue_date": "March 26, 2026",
            "customer_name": "Acme Corp",
            "line_items": build_line_items_html(items),
            "subtotal": "1,500.00",
            "tax": "150.00",
            "total": "1,650.00",
            "due_date": "April 30, 2026",
        },
        "email": {
            "to": "billing@acme.com",
            "subject": "Invoice INV-2026-0042 from Your Company",
        },
        "options": {"engine": "quality"},
    },
)

print("PDF URL:", response.json()["data"]["download_url"])

PHP (Laravel)

$response = Http::withToken('sk-your-api-key')
    ->post('https://pdf.funbrew.cloud/api/pdf/generate-from-template', [
        'template' => 'invoice',
        'variables' => [
            'invoice_number' => 'INV-2026-0042',
            'issue_date' => 'March 26, 2026',
            'customer_name' => 'Acme Corp',
            'line_items' => $this->buildLineItemsHtml($items),
            'subtotal' => '1,500.00',
            'tax' => '150.00',
            'total' => '1,650.00',
            'due_date' => 'April 30, 2026',
        ],
        'email' => [
            'to' => 'billing@acme.com',
            'subject' => 'Invoice INV-2026-0042 from Your Company',
        ],
        'options' => ['engine' => 'quality'],
    ]);

$downloadUrl = $response->json('data.download_url');

Step 3: Batch Processing for Monthly Billing

For end-of-month billing runs, use the batch API to generate all invoices at once.

const response = await fetch("https://pdf.funbrew.cloud/api/pdf/batch", {
  method: "POST",
  headers: {
    "Authorization": "Bearer sk-your-api-key",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    items: customers.map(c => ({
      type: "template",
      template: "invoice",
      variables: buildInvoiceVars(c),
      filename: `invoice-${c.id}-202603.pdf`,
      email: {
        to: c.email,
        subject: `Invoice for ${c.name} - March 2026`,
      },
    })),
  }),
});

const { data } = await response.json();
console.log(`Generated ${data.results.length} invoices`);

Get Notified with Webhooks

Want a Slack notification when invoices are generated? Set up a webhook URL in the dashboard. You'll receive a POST request like this:

{
  "event": "pdf.generated",
  "data": {
    "filename": "invoice-42-202603.pdf",
    "file_size": 48210,
    "download_url": "https://pdf.funbrew.cloud/dl/abc123..."
  }
}

Summary

Automating invoice PDF generation with an API eliminates manual work and errors:

  1. Create a template once, reuse forever
  2. One API call = PDF + email delivery
  3. Batch API for monthly billing runs
  4. Webhooks for completion notifications

Get started with the free plan (30 PDFs/month). Build your invoice template in the editor and test it in the Playground.

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