Zendesk is expensive as hell. Freshdesk I feel does even more, and is more or less completely free, if you only have a few agents. Even then, it's cheap. And, they have an API!
You can preview my Freshdesk at http://support.imperium42.com - feel free to make a ticket to test the system.
You could link their tickets via email. You could make a ticket via playfab by simply sending an email to their freshdesk email. All emails are converted to tickets. You can parse things by certain keywords. So it's fairly easy to integrate, from a broad perspective.
