I've never heard of anyone insuring the shipping cost. He should ONLY be insuring the contents of the box. Sending Priority International: Priority Mail International® Insurance* Uninsured Priority Mail International parcels include indemnity against loss, damage, or missing contents. Coverage is calculated based on the weight of the parcel or the value of the contents, whichever is less. You can purchase insured mail service if available for an additional fee. Additional insurance limits vary by country. When international insurance is purchased, it replaces indemnity coverage. Individual country prohibitions and restrictions apply.
More info at:
https://www.usps.com/ship/insurance-and-extra-services.htm Personally, I think I would back out. A seller that ignorant on insurance is probably just as ignorant on how to package an M70.