To update the weight units for the shipping you will need to update this line of code on the confirm and pp_confirm pages:
$Shipping_PackageArray[$Shipping_PackageIndex]["WeightUnits"] = "lb";
$Shipping_PackageArray[$Shipping_PackageIndex]["WeightUnits"] = "oz";
You may also want to update the label for this field in your admin area on the products insert and update pages. In the settings you will need to enable shipping with the checkbox first, then enter all of the details. When you get to either of the confirm pages you should see the shipping charge that is returned by USPS. If you have altered or updated the confirm or pp_confirm page and have this problem you may need to revert back to the original version to get it working.