I agree. If the post office was willing to take the package and charge ply30 for the shipping and insurance, they should hold most (if not all) of the responsibility. Millions of people ship through the postal service every day. The bulk of whom (unless they are businesses) are not experts in packaging items. However, that IS what the post office IS in business to do. Ship things. If they are to be the ones to determine whether or not an insurance claim is justified, then they need to be explicit on what standards they will accept when an item is packaged. - And they should not agree to insure an item if they aren't willing to honor the insurance.
I'm interested in hearing what the post office' response is.