curl --request POST \
  --url 'https://api.stateset.com/api/v1/ar/invoices/{invoice_id}/dunning' \
  --header 'Authorization: Bearer YOUR_API_KEY' \
  --header 'Content-Type: application/json' \
  --data '{
  "letter_type": "string",
  "sent_by": null
}'

{
  "activity_type": "string",
  "contact_method": null,
  "contact_result": null,
  "created_at": "string",
  "customer_id": "string",
  "dunning_letter_type": null,
  "id": "string",
  "invoice_id": "string",
  "notes": null,
  "performed_by": null,
  "promise_to_pay_amount": null,
  "promise_to_pay_date": null
}