the error was in the shipping server behavior on the conform page.
i changed line:
212:
$Shipping_PackageArray[$Shipping_PackageIndex]["PackageWeight"] = "".$mdacCart->DisplayInfo("") ."";
to specify the total weight column:
$Shipping_PackageArray[$Shipping_PackageIndex]["PackageWeight"] = "".$mdacCart->DisplayInfo("TotalWeight") ."";