You can auto add Shipsurance with a rule based on an order total, thanks! However if you don't add the amount to insure then nothing is charged and no insurance is applied to the order. We always insure for the same amount as on the order total, and when you manually add Shipsurance this amount is filled in automatically. So the same logic should be use with the automation rule.
Add: If there was a way to alert that Shipsurance is on an order with no amount applied that could also be helpful.