The indie hacker messaging stack:
what email, SMS and WhatsApp really cost at 0, 1k and 10k users

const res = await fetch('https://api.unlimitedmessaging.app/message', {
  method: 'POST',
  headers: {
    Authorization: `Bearer ${process.env.UM_API_KEY}`,
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({
    "to": {
      "email": "[email protected]",
      "phone": "+447700900123"
    },
    "subject": "Your Acme receipt",
    "text": "Payment received, thank you! Your receipt is in your account.",
    "channels": [
      {
        "channel": "EMAIL"
      },
      {
        "channel": "WHATSAPP"
      },
      {
        "channel": "SMS"
      }
    ]
  }),
});

Add Unlimited Messaging as the messaging layer of this app. Install @unlimited-messaging/sdk or use fetch, and read UM_API_KEY from the server environment only. Create a server-side notify(user, event) helper that calls POST https://api.unlimitedmessaging.app/message with Authorization: Bearer UM_API_KEY. Body: to: { email: user.email, phone: user.phone } (E.164), subject, text, channels: one entry per channel the user opted into among { channel: 'EMAIL' }, { channel: 'WHATSAPP' }, { channel: 'SMS' }. Use it for: welcome on signup (email + WhatsApp), payment receipt (email), important alert (WhatsApp, SMS if no WhatsApp). Add a notification preferences screen with the three channels. Log failures and never block the user flow. Reference: https://unlimitedmessaging.app/llms-full.txt