Dimensional weight and international shipping

Why do we say, "For international orders, you must arrange your own shipping"?
Simply, we have had too many issues with both package forwarding services (like myus) and with direct carriers like Fedex and DHL. Overcharges have cost us more than the sale price (so we lost money to sell a boat kit).
Walk through a shipping exercise with us. 
All shippers use a formula called "dimensional weight." Go to this Fedex calculator: http://www.fedex.com/in/tools/dimweight.html.  
As an example, enter the weight and dimensions (IN INCHES) of a box containing 6 Expandacraft parts (57 x 24 x 24 inches, 75 pounds). Calculator shows charges for 237 pounds, though actual weight is only 75 lbs. That means you will be charged shipping for a package weighing 237 lbs, even though actual weight is only 75 lbs!!! 
Do this calculation for each package. When getting shipping quotes, use DIMENSIONAL WEIGHT of each package for accurate costs.
We always tell customers to get a FIRM shipping quote before booking international shipping because standard charts are misleading and do not account for dimensional weight of oversize packages.