Why don’t you re-consider this from the perspective of shipping things yourself and making claims yourself.
You would prepare the package (at your cost) and provide it to the carrier (USPS was your example). Assume that you have insurance.
When the lost package is reimbursed, it will be for your invoiced cost.
You will not get the packaging cost for the freight cost reimbursed.
When you ship it a second time, pay the for the packaging and for the freight cost a second time along with the cost for the replacement item. Obviously not a good financial situation for you to reship, but better than an angry customer. You would instead have needed to return their payment, so you would be in a negative position rather than a reduced profit.
This is pretty much what Amazon does, but with their standard process, the net earnings from the sale are refunded in addition to the item cost.
Now, if you stopped here and never reshipped the product, you would have nothing to complain about.
However, your customer would not be happy and you would need to refund them. You would still take the hit.
You need to keep in mind that shipping and handling costs are sunk costs. If you had referral fee cost with Amazon for FBA, you get most of those back, but since it is MCF, that is not their problem.
Amazon is doing exactly what they committed to do. However, they are not covering the fulfillment costs you were hoping they would cover. That is within a reasonable policy.
You will not convince them to do otherwise.