Hello, WallyB.

I'm glad to inform you that investigation Error 401: deleted_client "The OAuth client was deleted" when trying to add app Invoice with Google Calendar has been resolved. And at this time, you should no longer experience any problem when invoicing with Google Calendar. I recommend doing a test and check if everything is working now on your end. 

For more information about your invoicing app, see this article: Invoice with Google Calendar FAQs.

Please know that I'm still here to help you further if you have more questions. Just add a comment. Wishing you all the best!