> if the buyer chooses not to pay for postal insurance
Postal insurance is something that may only be purchased by the sender(seller), and any claim on that insurance need be made by whomever is in possession of the original mailing receipt, and, if awarded, is collected by the that person. The claimant may have to produce proof of value. IMO, not much protection for the receiver(buyer)...
https://www.usps.com/help/claims.htm
fwiw