<?php echo WA_eCart_FormatNumber($yourCartName->TotalColumn('TotalWeight'), true, 1); ?>
where "yourCartName" is the name of your eCart object.
I tried to find that code or something similar doing a find on the confirm page and searching the source of all open documents and could not find anything. I then searched things like FormatNumber, TotalWeight to see what I could find and I found this.
$Shipping_PackageArray[$Shipping_PackageIndex]["PackageWeight"] = "".WA_eCart_FormatNumber($eCartHomesteadHomeUse->DisplayInfo('TotalWeight') + $addedWeight, true, 1) ."";