This is the basic formula we follow for shipping:
((cost per oz.) X (product wt. + package wt.)) + (package cost) + (handling cost)
You can specify custom rates for Domestic vs. International shipping only on a cost-per-oz. (or gram if you prefer :^) basis. At present, you cannot further subdivide International rates into two or more regions, nor can you specify flat-rate shipping regardless of order quantity that is different for Domestic vs. Int'l. The closest you can do is set your custom Domestic rate to 1.00, set your Int'l rate to some multiple of Domestic (e.g. if your Int'l rate will be double your Domestic rate, set the Int'l rate to 2.00), then set the "weight" of each product unit as equivalent to your Domestic shipping cost for each unit.