How do i define permissions for my Billing or Usage export buckets for read-only access by Teevity ?

In the Google Console, select the project with your export bucket(s) and click on "Storage browser" in the left navbar.

Click on the 'triple dot' button at the end of the bucket line then click on "Edit bucket permissions" on the menu.

Add the 'Teevity ServiceAccount email' as a User with Reader permissions
Teevity ServiceAccount email address 985668681564-9257rkc0bnb09l6e5228bb1ornpt0942@developer.gserviceaccount.com

A pop-up confirms that your ACL have been updated