Windowed and Windowless Letters can be sent by submitting only a PDF URL via the API. 
  curl -X POST "https://api.thanks.io/api/v2/send/windowlessletter" \
    -H "Content-Type: application/json" \
    -H "Authorization: Bearer YOUR_API_KEY" \
    -d '{ 
      "pdf_url": "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf",
      "recipients": [
        {
          "name": "Current Resident",
          "address": "123 Main Street",
          "city": "Fake City",
          "province": "NY",
          "postal_code": "55555",
          "country": "US",
          "custom1": "Example Custom 1",
          "custom2": "Example Custom 2",
          "custom3": "Example Custom 3",
          "custom4": "Example Custom 4"
        },
        {
          "name": "Jane Doe",
          "address": "456 Another St",
          "city": "Fake City",
          "province": "NY",
          "postal_code": "55555",
          "country": "US"
        }
      ]
    }'