Live shipping rate lookups are only available for US merchants using the US Postal Service, and that postage lookup is the only purpose of the "Shipping Origin" zipcode.
Otherwise, you will only need to select your own country as the Shipping Origin, which determines whether we apply your custom Domestic or International rate to the shipping calculation for a given order, and devise a custom shipping calculation method such as the examples given here:
http://www.e-junkie.com/ej/help.shipping.htm
You may also find it helpful to know the basic formula we follow when calculating shipping charges:
(rate X (total product weights + container weights)) + (container cost) + (handling cost)
Note that you can "game" the formula by applying non-literal figures for various settings, so the formula adds up the way you want.