You may encounter the error "Insurance is not available with this service/packaging combo" when trying to add insurance to a USPS shipment that does not include some form of delivery confirmation.
Most USPS shipments (domestic and international) can be insured in $100 increments with Shipsurance. However, Shipsurance insurance is only available when a form of delivery confirmation is on the shipment (i.e. tracking, delivery confirmation, signature confirmation, adult signature confirmation).
To add insurance to a shipment, you will need to select a service that includes delivery confirmation or add delivery confirmation to the shipment. You can add a Confirmation option on the READY TO SHIP page, from the "Additional Shipment Options" Confirmation drop-down menu.