The shipping you add at the product level are applied no matter what and have no effect on the checkout.
the shipping rules you define in the Admin -> Settings -> Shipping are applied at checkout and will effect where an order can be shipped to
if you have only a US rule, there will be a message to call the store if another shipping country is selected.
The shipping rules defined in the admin are applied on top of the shipping you set at the product level.
when you set up a rule, you select the destination country for that rule, the provider and delivery type
you can have a different provider and different delivery type for each country.
USPS is one of the available providers.
the shipping rules do not have a setting for additional insurance.
that should probably be added at the item level.