Delivered - Add an order status of delivered. If you're already polling to get data on the carrier status to show the delivered icon in the UI, it shouldn't be hard to write a line to push it to the Shipstation order table in MySQL to have the status 'Delivered'. For integrations and api users, this would be super ideal. Plus add a webhook for this.
Webhooks are extremely lacking, and then for the webhooks, a resource_url is not useful. Look at stripe and how they handle their webhooks. A lot could be learned from how they handle their webhooks, The current implementation is extremely inefficient for integrations.