Arvind Anbuvel

Figured it out. It was to do with the spacing on the json request. 

The json payload must have one space before and after ":" and no other space or tabs anywhere else. See example below: 

{"id" : "XXX", "amount" : "4567.89", "currency" : "aud", "reason" : "product_not_received"}

The payload fields are not ordered and spaces not trimmed. Zendesk really should have taken care of simple things like this.