From version 1.1.1, when the callback URL is requested by Snipcart it will trigger the event on_snipcart_callback (as well as continue do stock level updates if enabled).

This event passes the order summary data, meaning that a custom action can be performed in response. For example, this event could be used to trigger the sending of an additional email, to add a customer's email to a mailing list or to log information.

An installable example package of how to utilise this event is available on github.