to avoid issues over international shipping issues (i live in uk, and have just had something sent to me by a forums member so i looked into this), if you list an approx weight for each item, then you/us can just run it through the usps rate calculator.
fwiw usps appears to be the cheapest way to send internationaly
edit - it is quite common for smaller, specialist suppliers, 2cooltek springs to mind - to pack the customers order then take it to the post office to find out how much it is, then modify the order and charge it.
otoh, if your feeling really clever, you could run some weights through the rate calculator, and then incorporate those figures directly