USPS has an auto-insurance up to $100 if you send it Express Mail, otherwise you have to decide to do it.
I add insurance on everything I ship if the sell price was over $20 and/or I don't have another one to ship as a replacement if this one gets lost or damaged. If an item is easily damaged (glass), then it gets insurance no matter what. Insurance is always for the amount of the item plus shipping. When I sell an item, my "shipped" price includes the insurance and delivery confirmation.
I have never had an item sent by me damaged or lost. I have received items that have had rips in the packages where it was lucky the item inside wasn't lost, stolen or damaged, but everything was okay. Usually this was due to the packaging (sticking a joystick assembly inside a non-padded manila envelope, for example)....