As a Wave customer, you have access to all Wave's capabilities, including our published APIs.
API Terms of Service
Use of the Wave API is subject to our Terms of Service. If you do not agree to these Terms, your sole remedy is to not use the API.
In Practical terms...
We encourage all Wave customers to optimize their efficiency using Wave. The whole of our published API is available to you as a Wave customer to build integrations for use in your own business, subject to the API Terms of Service.
When you build an integration to Wave for your own use, you will create an Application in our Developer Portal, and be issued a permanent* Access Token for authentication. This Access Token will give your application full access to all areas of your Wave account; it should be kept secret, and not shared with third parties.
Use of our APIs to connect to your own Wave account using a permanent Access Token is free of charge.
* "Permanent" access tokens are issued with a long expiry date, typically 3 or more years in the future. Wave may also force replacement of a "Permanent" access token earlier if there is any concern that the security of your token may have been compromised. Accordingly, you should expect - eventually - to update the Access Token within your application.