To make integrating with the FreshBooks API easier we've made a Postman collection that you can populate with your own application credentials and environment variables.
The collection is publicly available at: https://documenter.getpostman.com/view/3322108/S1ERwwza
Once you have registered your application you'll get credentials that you can use in Postman with the Oauth2 authorization flow. This will allow you to easily get a new bearer token that can be added to the header of all calls automatically. If you do this at the collection level, then all the individual requests will inherit the parent auth token
We have a "sandbox env" environment that matches up to all the variables in the individual calls. We've made attempts to auto-populate these item specific variables after a POST call or a GET all. In the case of the POST calls, the returned item id will be stored so an individual GET call will have the required ID. In the case of a GET all items, we'll take the response ID of the first item and populate the individual item ID for subsiquent individual GET calls.