How to Automate Invoice Generation in Your SaaS (2026)

The Invoice Problem Every SaaS Faces

Your SaaS is growing. Customers are paying through Stripe. But someone — usually you — is still manually creating invoices in Google Docs or Word. Or you're paying $50+/month for an invoicing tool that does way more than you need.

There's a better way: generate invoices programmatically with an API. When a Stripe payment succeeds, call the API, get a professional PDF, and email it to the customer. Fully automated, zero manual work.

Architecture: Event-Driven Invoice Generation

The flow is simple:

1. Stripe webhook fires — invoice.payment_succeeded event
2. Your server receives it — validates the signature, extracts payment details
3. Call an invoice API — send the line items, customer info, and payment metadata as JSON
4. Get a PDF back — professional, branded, ready to send
5. Email or store it — attach the PDF to a transactional email or save to S3

This entire flow runs in under 2 seconds. No human involvement. No missed invoices.

Setting Up the Stripe Webhook

First, create a webhook endpoint that listens for payment events:

@app.post("/webhooks/stripe")
async def stripe_webhook(request: Request):
    payload = await request.body()
    sig = request.headers.get("stripe-signature")
    event = stripe.Webhook.construct_event(
        payload, sig, WEBHOOK_SECRET
    )
    if event["type"] == "invoice.payment_succeeded":
        invoice_data = event["data"]["object"]
        await generate_and_send_invoice(invoice_data)
    return {"status": "ok"}

Generating the Invoice PDF

Transform the Stripe event data into an invoice API request:

async def generate_and_send_invoice(stripe_invoice):
    invoice_json = {
        "from": {"name": "Your Company", "email": "[email protected]"},
        "to": {"name": stripe_invoice["customer_name"],
               "email": stripe_invoice["customer_email"]},
        "items": [
            {"description": line["description"],
             "quantity": line["quantity"],
             "unit_price": line["amount"] / 100}
            for line in stripe_invoice["lines"]["data"]
        ],
        "currency_symbol": "$",
        "invoice_number": stripe_invoice["number"],
        "template": "modern"
    }
    # Call DocuMint API
    resp = httpx.post(
        "https://documint.anethoth.com/api/v1/invoice",
        json=invoice_json,
        headers={"X-API-Key": DOCUMINT_KEY}
    )
    pdf_bytes = resp.content
    await send_invoice_email(
        stripe_invoice["customer_email"], pdf_bytes
    )

Why Not Use Stripe's Built-in Invoices?

Stripe does have invoicing, but it's limited:

• No custom templates — Stripe invoices look like Stripe invoices. You can add a logo, but the layout is fixed.
• No custom fields — Need a PO number, tax ID, or custom footer? Limited options.
• PDF generation costs extra — Stripe charges for invoice finalization at scale.
• Tied to Stripe — If you ever switch payment processors, your invoicing breaks.

An API-based approach gives you full control over the design, fields, and delivery — decoupled from any payment provider.

Handling Edge Cases

Production invoice generation needs to handle:

• Failed webhook deliveries — Stripe retries, so make your handler idempotent. Check if an invoice was already generated for this payment.
• Multi-currency — Pass the currency code from Stripe. Format amounts appropriately (¥ has no decimals).
• Tax compliance — Different regions require different tax line items. Build this into your invoice JSON structure from day one.
• Credit notes and refunds — Listen for credit_note.created events and generate matching credit note PDFs.

The Result

With this setup, every payment automatically produces a professional invoice. Your customers get PDFs instantly. Your accountant gets consistent, machine-readable documents. And you never manually create an invoice again.

DocuMint handles the PDF generation — send JSON, get a PDF in under 500ms. Free tier includes 10 invoices/month, enough to set this up and test it end-to-end before you need a paid plan.