Stamps.com gives discounts on postage. So they use their own scales. Like if u put a nickel on it it will only read 2 grams not 5. So the 3 grams would be your discount
the problem comes in when your piece of mail is weighed by the post office to be more then what you paid for - and it auto charges your stamps.com account or method of payment for the difference.
I have a friend who works at the post office and he's told me to get this scale and stamps.com to get the discount. When they see it's from stamps.com they know you've paid what needs to be paid.