curl -X PATCH https://api.v2.dealmachine.com/v1/mail/settings \
  -H "Authorization: Bearer dm_sk_live_xxx" \
  -H "Content-Type: application/json" \
  -d '{
    "default_postcard_size": "6x9",
    "contact_frequency_limit_days": 30
  }'

{
  "data": {
    "default_postcard_size": "6x9",
    "default_sender_name": "Acme Real Estate",
    "contact_frequency_limit_days": 30,
    "default_qr_code_url": "https://acme.com/offer",
    "default_signature_image_url": "https://atlas.s3.amazonaws.com/signatures/abc.png"
  }
}