I want to use flat shipping rates for my store and also want to set one rate for orders in the US & another rate for International orders.
Is there any way to do this on a per item basis?
Store doesn't support custom shipping rates for each item out of the box. It does support a "handling" charge for each product, but this is not specified on a per-country basis.
If you need to implement different charges for each product, for each country, then you would need to write a custom shipping extension. You can use the store_order_shipping_methods
hook:
https://exp-resso.com/docs/extension_hooks.html#shipping_hooks
If you are using the default shipping plugin you should be able to create a rule for the US and a rule below it with the country set to "any". In Store with the default shipping method rules are evaluated from top to bottom so people in the US would get the US rate and everyone else would get the rate set for "any".