I think that the issue you are having is paying someone who has invested the odd thousand bucks for legit scales. If you want to legitimately weight and scale your car (corner balancing), you must use the right equipment. Weighing your car really is a waste of time unless you are corner balancing it at the same time.
At the end of the day, you will never know if the bathroom scale is being truthful. My wife says hers always lies anyway.
Corner balancing is usually anywhere from 50 to 100 bucks, depending on the shop.
You will be amazed at how well a properly corner balanced car will handle.