When purchasing insurance for mailing a package, does it guarantee it will arrive at the addressee or does it guarantee it will arrive in its original condition?

If a package that is not insured or have delivery confirmation, there is really no way of knowing if the sender actually sent the item, right?

So bottom line, if a sender says he sent an item, and it isn't received, the USPS is at fault with no recourse?

Your opinion is requested...thanks