What is the shipping option that you have selected in your admin area for PowerStore? There might be a more appropriate one for you to select based on your requirements.
As for the weight being in ounces the PowerStore is hard coded to work with pounds. If you would like to get the proper weight for your items in ounces without modifying the store with eCart you could just take the ounce weight and divide by 16 to get the pound decimal value. For instance, a 5oz item would be about .31 pounds. Other than this you would need to make use of eCart to alter the default weight unit.