Why do you think the shipping forms have separate sections for item value and insurance value then? I inquired at the post office about this before and the two values do not necessarily have to be related.

If it is insured and gets lost or damaged you are indeed entitled to compensation under the terms of the insurance. That's why you pay the insurance premium. The item value is for import/customs purposes. You can choose to under- or overinsure I suppose, depending on your preferences. It also depends on the country where you are importing and their rules.