Google App Scripts UI for Faxage API

We are transitioning from a traditional fax machine to an online fax service. The online fax service (faxage) has an api, which is documented at [login to view URL] and [login to view URL]) Use Case: Users send and receive faxes via faxage's fax service. Users need to be able to view the fax history for sent and received faxes. Users also need to be able to view/download faxes they sent or received. When users send faxes, faxage creates a fax confirmation page confirming the result (i.e. Success or Failure). For sent faxes, users must be able to view/download the fax confirmation page. Attached is an example history page from the faxage website. In short, we are moving the fax history functionality from faxage to google drive/doc. This app will enable the user to view the fax history directly from google drive/docs. Specifications: The goal of the project is to create an integration between faxage and Google Drive. The integration must be built using Google App Scripts ([login to view URL]). Either a "web app" or" add-on" is acceptable. The developer must create a google drive (or docs) web app or add-on that will display fax history information stored in faxage. All of the fax history data is accessible from the faxage API. For sent faxes only, the user should also be able to view the confirmation page generated by faxage. An example dialog for sent faxes and received faxes is attached. The example dialogs only provide an outline and do not include any styling. The developer must design and style the dialog in accordance with google's style specifications found here [login to view URL] The dialog's style should look like the same as the built-in google dialogs. In other words, the dialogs should be styled the same as the native google drive dialogs. The dialog should look like they are part of the standard google drive/docs products. I am unsure how to best handle authentication. Developer will be provided API keys and recommend the best authentication method. User's should NOT be repeatedly prompted for credentials.