The developer's guide to sending physical letters from code.

The developer's mental model: How cloud-to-letterbox architecture works

The 3-step plan: From zero to your first dispatched letter in 5 minutes

Technical blueprint: Sending a physical letter via cURL and Node.js

Terminal — bash
curl -X POST https://api.letterapi.uk/v1/letters \
  -H "Authorization: Bearer test_sec_9938b827e8a9f02938472910" \
  -H "Content-Type: application/json" \
  -H "Idempotency-Key: job_ord_849204810" \
  -d '{
    "recipient": {
      "name": "Sarah Jenkins",
      "company": "Apex Financial Ltd",
      "address_line1": "100 Bishopsgate",
      "address_line2": "Floor 14",
      "city": "London",
      "postcode": "EC2N 4AG",
      "country": "GB"
    },
    "template_html": "<html><body><h1>Statutory Annual Review</h1><p>Dear Sarah, your account statement is ready.</p></body></html>",
    "postage_class": "first_class",
    "color": true,
    "double_sided": false
  }'

src/send-letter.ts — typescript
import axios from 'axios';
​
interface LetterResponse {
  id: string;
  status: 'created' | 'validating' | 'queued_for_print';
  recipient: {
    formatted_address: string[];
    paf_verified: boolean;
  };
  cost: {
    currency: string;
    print: number;
    postage: number;
    total: number;
  };
  estimated_delivery: string;
  created_at: string;
}
​
export async function sendPhysicalLetter(): Promise<LetterResponse> {
  const payload = {
    recipient: {
      name: 'Alexander Wright',
      company: 'Pinnacle Logistics UK',
      address_line1: '45 Oxford Street',
      city: 'Manchester',
      postcode: 'M1 4EE',
      country: 'GB'
    },
    template_html: `
      <!DOCTYPE html>
      <html>
      <head>
        <style>
          @page { size: A4; margin: 20mm; }
          body { font-family: 'Helvetica Neue', Arial, sans-serif; color: #0f172a; line-height: 1.5; }
          .address-block { margin-top: 15mm; margin-bottom: 25mm; font-size: 11pt; }
          .brand { font-size: 18pt; font-weight: bold; color: #0284c7; border-bottom: 2px solid #0284c7; padding-bottom: 8px; }
          .content { margin-top: 20px; font-size: 10.5pt; }
        </style>
      </head>
      <body>
        <div class="address-block">
          <strong>Alexander Wright</strong><br/>
          Pinnacle Logistics UK<br/>
          45 Oxford Street<br/>
          Manchester<br/>
          <strong>M1 4EE</strong>
        </div>
        <div class="brand">PINNACLE AUDIT VERIFICATION</div>
        <div class="content">
          <p>Dear Mr. Wright,</p>
          <p>This letter confirms that your annual compliance audit has been formally registered with the UK Department for Business and Trade.</p>
          <p>Enclosed in your online portal is your verified digital certificate.</p>
        </div>
      </body>
      </html>
    `,
    postage_class: 'first_class',
    color: true,
    double_sided: false,
    metadata: {
      internal_audit_id: 'AUD-2026-9921'
    }
  };
​
  const response = await axios.post<LetterResponse>(
    'https://api.letterapi.uk/v1/letters',
    payload,
    {
      headers: {
        'Authorization': `Bearer ${process.env.LETTERAPI_API_KEY}`,
        'Content-Type': 'application/json',
        'Idempotency-Key': `dispatch_${Date.now()}`
      }
    }
  );
​
  console.log(`Letter created successfully! ID: ${response.data.id}`);
  console.log(`PAF Verified: ${response.data.recipient.paf_verified}`);
  console.log(`Estimated Delivery: ${response.data.estimated_delivery}`);
  return response.data;
}

Understanding Royal Mail DL window envelope geometry

Listening to real-time delivery webhooks: Building reactive systems

The unit economics: Transparent, pay-as-you-go pricing

The failure stakes: What happens when teams rely on manual office post

The success transformation: Complete postal automation through code

Automating document generation pipelines: Headless Chrome vs Weasyprint vs raw HTML

Security architecture: API authentication, HMAC request signatures, and credential rotation

Handling Royal Mail delivery classes: First Class vs Second Class vs Tracked 24/48

Multi-tenant letter dispatch: Metadata tagging and sub-account billing for SaaS platforms

Handling international mailings: Royal Mail International Standard and Tracked services

The developer quick-reference: Best practices for high-velocity postal engineering

The developer's deployment checklist: Going from sandbox to production in 10 minutes