By "flat rate charges", I infer you mean that you used our suggested technique of having the per-oz. rate set as 1.00, with the per-item shipping cost being set as the "weight" of each item (e.g., $1.00 per oz. X 17.49 oz./item = $17.49 shipping per item). In this case, you would set the International per-oz. rate as some multiple of your Domestic rate. E.g., if your rates to Canada run, say, 20% higher than your Domestic rate, then your Int'l rate is 120% (or 1.20 times) of your Domestic rate, so you would set your Int'l per-oz. rate to 1.20. 25% higher would be 1.25, double the rate would be 2.00, etc.