You could try replacing the code:
$Shipping_PackageArray[$Shipping_PackageIndex]["eCart_Quantity"] = (($Shipping_Packaging == 1)?$guitartables->DisplayInfo("Quantity"):1);
$Shipping_PackageArray[$Shipping_PackageIndex]["IsPackage"] = "true";
$Shipping_PackageArray[$Shipping_PackageIndex]["Quantity"] = "1";
$Shipping_PackageArray[$Shipping_PackageIndex]["WeightUnits"] = "".strtoupper($guitartables->WeightUnits)."LBS" ."";
$Shipping_PackageArray[$Shipping_PackageIndex]["PackageWeight"] = "".WA_eCart_FormatNumber($guitartables->DisplayInfo('Weight'), true, 1) ."";
with:
$Shipping_PackageArray[$Shipping_PackageIndex]["eCart_Quantity"] = (($Shipping_Packaging == 1)?$guitartables->DisplayInfo("Quantity"):1);
$Shipping_PackageArray[$Shipping_PackageIndex]["IsPackage"] = "true";
$Shipping_PackageArray[$Shipping_PackageIndex]["Quantity"] = (($Shipping_Packaging == 1)?$guitartables->DisplayInfo("Quantity"):1);
$Shipping_PackageArray[$Shipping_PackageIndex]["WeightUnits"] = "".strtoupper($guitartables->WeightUnits)."LBS" ."";
$Shipping_PackageArray[$Shipping_PackageIndex]["PackageWeight"] = "".WA_eCart_FormatNumber($guitartables->DisplayInfo('Weight'), true, 1) ."";