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Invoice and billing emails are transactional messages that carry legal and financial significance. They serve as receipts, payment reminders, and accounting records. Unlike most transactional emails, invoices often need to meet specific legal formatting requirements that vary by jurisdiction. A missed or spam-filtered invoice can lead to late payments, revenue delays, and frustrated customers. This guide covers legal requirements, content structure, attachment handling, and delivery best practices.

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Types of Billing Emails

Email TypeTriggerClassification
Invoice / ReceiptPayment processed successfullyTransactional
Payment reminderUpcoming payment due dateTransactional (if related to an existing agreement)
Failed payment noticePayment method declinedTransactional
Subscription renewalRecurring billing cycle completedTransactional
Plan change confirmationUser upgrades or downgradesTransactional
Payment method expiringCard approaching expirationTransactional
Refund confirmationRefund issuedTransactional
Promotional pricing offerDiscount or upsellMarketing — requires unsubscribe
All billing emails related to an existing business transaction are transactional and do not require an unsubscribe link. However, emails promoting upgrades, new plans, or discounts are marketing and must comply with CAN-SPAM, GDPR, and other regulations. See Transactional vs Marketing.
Invoice formatting requirements vary by country. Always verify the requirements for the jurisdictions where your customers are located.

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Common Required Fields

Most jurisdictions require invoices to include:
1

Seller identification

Your business name, address, and tax identification number (e.g., VAT number in the EU, EIN in the US).
2

Buyer identification

Customer name or business name, and address for B2B invoices.
3

Invoice number

A unique, sequential identifier. Many jurisdictions require invoice numbers to be sequential without gaps.
4

Invoice date

The date the invoice was issued.
5

Line items

Description of goods or services, quantity, unit price, and line total.
6

Tax breakdown

Tax rate applied, tax amount, and whether prices are inclusive or exclusive of tax.
7

Total amount

Net amount, tax amount, and gross total. Currency must be clearly stated.
8

Payment terms

Due date, accepted payment methods, and any late payment penalties.

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Regional Requirements

RegionKey Requirements
EU (VAT)VAT number (seller and buyer for B2B), VAT rate per line item, “reverse charge” notation for cross-border B2B sales
USLess prescriptive — business name, address, EIN for B2B, and state sales tax if applicable
UKVAT number, VAT rate, business registration number
CanadaGST/HST number, tax breakdown by province
AustraliaABN (Australian Business Number), GST amounts
This is general guidance, not legal advice. Consult with an accountant or legal professional to ensure your invoices meet the specific requirements of your operating jurisdictions.

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Email Content and Structure

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Invoice Email Template

<div style="font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif; max-width: 600px; margin: 0 auto; color: #1a1a1a;">

  <!-- Header -->
  <div style="padding: 24px 0; border-bottom: 1px solid #e5e5e5;">
    <h2 style="margin: 0;">Invoice #{{invoiceNumber}}</h2>
    <p style="color: #6b6b6b; margin: 8px 0 0;">
      {{invoiceDate}} · {{currency}} {{totalAmount}}
    </p>
  </div>

  <!-- Status -->
  <div style="background-color: #f0fdf4; border-radius: 8px; padding: 12px 16px; margin: 24px 0;">
    <span style="color: #166534; font-weight: 600;">Payment successful</span>
    <span style="color: #6b6b6b;"> · {{paymentMethod}}</span>
  </div>

  <!-- Line Items -->
  <table style="width: 100%; border-collapse: collapse; margin: 24px 0;">
    <thead>
      <tr style="border-bottom: 2px solid #e5e5e5;">
        <th style="text-align: left; padding: 8px 0; font-size: 13px; color: #6b6b6b; font-weight: 600;">Description</th>
        <th style="text-align: right; padding: 8px 0; font-size: 13px; color: #6b6b6b; font-weight: 600;">Amount</th>
      </tr>
    </thead>
    <tbody>
      <tr style="border-bottom: 1px solid #e5e5e5;">
        <td style="padding: 12px 0;">
          {{planName}} — {{billingPeriod}}<br>
          <span style="color: #6b6b6b; font-size: 14px;">{{periodStart}} to {{periodEnd}}</span>
        </td>
        <td style="text-align: right; padding: 12px 0;">{{lineAmount}}</td>
      </tr>
    </tbody>
  </table>

  <!-- Totals -->
  <table style="width: 100%; margin-bottom: 24px;">
    <tr>
      <td style="padding: 4px 0; color: #6b6b6b;">Subtotal</td>
      <td style="text-align: right;">{{subtotal}}</td>
    </tr>
    <tr>
      <td style="padding: 4px 0; color: #6b6b6b;">Tax ({{taxRate}}%)</td>
      <td style="text-align: right;">{{taxAmount}}</td>
    </tr>
    <tr style="border-top: 2px solid #1a1a1a;">
      <td style="padding: 12px 0; font-weight: 600;">Total</td>
      <td style="text-align: right; font-weight: 600;">{{currency}} {{totalAmount}}</td>
    </tr>
  </table>

  <!-- Seller Info -->
  <div style="padding: 16px; background-color: #f9fafb; border-radius: 8px; font-size: 13px; color: #6b6b6b; line-height: 1.6;">
    <strong style="color: #1a1a1a;">{{businessName}}</strong><br>
    {{businessAddress}}<br>
    Tax ID: {{taxId}}
  </div>

  <!-- CTA -->
  <div style="text-align: center; margin: 32px 0;">
    <a href="{{invoiceUrl}}"
       style="background-color: #6366F1; color: #ffffff; padding: 12px 32px;
              text-decoration: none; border-radius: 6px; font-weight: 600;">
      View Invoice
    </a>
  </div>

  <p style="color: #6b6b6b; font-size: 13px; text-align: center; line-height: 1.6;">
    Questions about this invoice? Contact
    <a href="mailto:billing@yourapp.com" style="color: #6366F1;">billing@yourapp.com</a>
  </p>
</div>

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Inline vs Attachment

There are two approaches to delivering the actual invoice: inline in the email body, or as a PDF attachment.
ApproachProsCons
Inline (HTML in email body)Immediate visibility, no extra step to view, works on mobileHarder to save/print, may not meet legal archival requirements
PDF attachmentProfessional format, easy to save/print/archive, meets legal requirementsLarger email size, some providers flag attachments, requires PDF generation
Link to hosted invoiceSmallest email size, always up-to-date, no attachment flagsRequires the user to click, link can expire, depends on your uptime
Use a combination: include a summary in the email body and provide a link to download or view the full invoice as a PDF: 
const response = await fetch('https://app.lettr.com/api/emails', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.LETTR_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    from: 'billing@mail.yourapp.com',
    to: customer.email,
    subject: `Invoice #${invoice.number} — ${invoice.currency} ${invoice.total}`,
    html: invoiceEmailHtml,
    text: invoiceEmailText,
    tags: ['invoice', 'billing'],
    metadata: {
      invoiceId: invoice.id,
      customerId: customer.id,
      amount: invoice.total,
    },
  }),
});
Include the invoice number and amount in the subject line. Customers and their accounting teams search for invoices by number and amount — making these searchable in the subject saves time.

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If You Attach a PDF

When attaching an invoice PDF, keep the file size under 1 MB and use Lettr’s attachment support: 
const fs = require('fs');

const pdfBuffer = await generateInvoicePdf(invoice);
const pdfBase64 = pdfBuffer.toString('base64');

const response = await fetch('https://app.lettr.com/api/emails', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.LETTR_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    from: 'billing@mail.yourapp.com',
    to: customer.email,
    subject: `Invoice #${invoice.number} — ${invoice.currency} ${invoice.total}`,
    html: invoiceEmailHtml,
    text: invoiceEmailText,
    attachments: [
      {
        filename: `invoice-${invoice.number}.pdf`,
        content: pdfBase64,
        contentType: 'application/pdf',
      },
    ],
    tags: ['invoice'],
  }),
});
Large attachments can trigger spam filters and slow delivery. Keep PDF attachments under 1 MB. If the invoice is larger (e.g., includes detailed line items or images), host the PDF and include a download link instead.

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Failed Payment Emails

Failed payment notices need special attention because they directly affect your revenue. A customer who doesn’t see the failed payment notice will churn unintentionally.

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Dunning Email Sequence

EmailTimingTone
1. Payment failedImmediately after failureInformational — “Your payment didn’t go through, here’s how to update it”
2. Reminder3 days after failureFriendly reminder — “Your payment is still pending”
3. Urgent notice7 days after failureWarning — “Your account will be downgraded in X days”
4. Final notice14 days after failureLast chance — “Your account has been downgraded, update payment to restore”
// Failed payment email
await fetch('https://app.lettr.com/api/emails', {
  method: 'POST',
  headers: {
    'Authorization': `Bearer ${process.env.LETTR_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    from: 'billing@mail.yourapp.com',
    to: customer.email,
    subject: 'Action required: Your payment could not be processed',
    html: failedPaymentHtml,
    text: failedPaymentText,
    tags: ['billing', 'payment-failed', `dunning-${attempt}`],
    metadata: {
      customerId: customer.id,
      dunningStep: attempt,
      failedAmount: invoice.total,
    },
  }),
});

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Failed Payment Email Content

  • Explain what happened — “We tried to charge your Visa ending in 4242 but the payment was declined”
  • Don’t speculate why — you rarely know the reason for the decline. Don’t say “insufficient funds” — just say the payment didn’t go through
  • Provide a direct link to update the payment method
  • State the consequence — what happens if the payment isn’t resolved (service downgrade, access loss)
  • Include a deadline — when the account will be affected

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Subscription Renewal Emails

For recurring billing, send a notification shortly before or immediately after each charge:
TimingContent
Before charge (3–7 days)“Your subscription renews on [date] for [amount]. No action needed.”
After chargeStandard invoice/receipt
Pre-charge notifications are required by some payment processors (Stripe recommends them) and reduce chargebacks. They also give customers a chance to update their payment method before it’s charged.

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Currency and Localization

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Formatting Currency

Always display currency amounts with the correct symbol, decimal separator, and thousands separator for the customer’s locale:
LocaleFormatExample
US/UK$1,234.56$1,234.56
Germany1.234,56 €1.234,56 €
Japan¥123,456¥123,456
SwitzerlandCHF 1’234.56CHF 1'234.56
function formatCurrency(amount, currency, locale) {
  return new Intl.NumberFormat(locale, {
    style: 'currency',
    currency: currency,
  }).format(amount);
}

// Examples
formatCurrency(1234.56, 'USD', 'en-US');  // "$1,234.56"
formatCurrency(1234.56, 'EUR', 'de-DE');  // "1.234,56 €"
Store amounts in the smallest currency unit (e.g., cents) to avoid floating-point issues. Convert to the display format only when rendering the email.

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Common Mistakes

Customers reply to billing emails with payment questions, refund requests, and disputes. Use a monitored billing@ address so these reach your finance or support team.
Many jurisdictions require sequential invoice numbering without gaps. Use a database sequence or auto-increment, not random IDs.
Displaying “1.234,56"toaUScustomer(or"1.234,56" to a US customer (or "1,234.56” to a German customer) causes confusion. Format amounts according to the customer’s locale.
A single “payment failed” email with no follow-up loses revenue. Implement a dunning sequence that escalates over 2 weeks before downgrading the account.
PDFs over 1 MB can trigger spam filters or slow delivery. Optimize your invoice PDF generation or host the file and send a download link instead.
Never include full card numbers, bank account numbers, or security codes. Show only the card type and last four digits.

Order Confirmation Emails

E-commerce order confirmations and receipts

Data Privacy in Email Metadata

Protecting sensitive information in email content and metadata

Deliverability Best Practices

Maximize inbox placement for your emails

CAN-SPAM Requirements

US email compliance requirements

Stripe Integration

Send payment emails directly from Stripe