Hello Andrew,
My application has been there for quite some time. Can you share the details that you used to approve your application?
If you can share the credentials of your application by email, I can try to make the package work with Esty OAuth 2 (Etsy API version 3);
The OAuth class already supports the automatic renewal of access tokens using refresh tokens. The base oauth_client_class uses PHP sessions to store receipts, so it only works when the user is present.
There is the file_oauth_client_class that is more suited to make API calls when the user is not present because access tokens and refresh tokens are stored in server-side files. There is a tutorial article for that here:
phpclasses.org/blog/package/7700/po
...
Would you like me to try to make it work with your application credentials, or do you prefer to try to make it work yourself?