For any order shipped as first class mail (the most economical option) insurance is not included. Priority Mail automatically covers $100 of insurance.
Insurance only covers lost or damaged goods. This is USPS's policy, not mine. If your package is marked as delivered but is not there, it was most likely stolen and would not be covered by insurance. Stolen packages are not covered by insurance. Your best bet to reduce package theft would be to purchase a signature confirmation.