How do you have the number of packages setup? If you have it set to one package for the entire order rather than have two packages of similar size and weight you will have one larger package. This could be what you are seeing with the increased price.
As for adding in the LBS you should not need to do this if your cart is set to LB for the weight. The default code that is created by eCart 5 for the weight unit looks like this:
$Shipping_PackageArray[$Shipping_PackageIndex]["WeightUnits"] = "".strtoupper($eCart1->WeightUnits)."S" ."";
You are taking the default weight unit and just adding an s to it, so if you had oz or lb as the weight unit you would end up just having an s added to the end. So the code should already work for this.