curl -X POST https://thepostalcompany.com/api/v1/letters/a1b2c3d4-e5f6-7890-abcd-ef1234567890/send \
  -H "Authorization: Bearer tpc_your_api_key"

const response = await fetch(
  "https://thepostalcompany.com/api/v1/letters/a1b2c3d4-e5f6-7890-abcd-ef1234567890/send",
  {
    method: "POST",
    headers: { "Authorization": "Bearer tpc_your_api_key" },
  }
);

const data = await response.json();
console.log(data.status); // "processing"

{
  "id": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
  "status": "processing",
  "balance_cents": 750
}

{
  "error": "Letter cannot be sent — current status is 'sent'"
}

{
  "error": "Insufficient balance",
  "balance_cents": 100,
  "letter_price_cents": 250
}

{
  "error": "Letter not found"
}