It's normal, but you can only charge what you actually weigh, and some subtle things will affect those weights. I remember a case a few years ago. A woman went to a jewelry store to wash a pearl necklace and lost a few grams when she got married. Later, the police intervened and found that when she came, she was weighed in a plastic bag, but when she came to pick it up, she was weighed separately, that is, the weight of the plastic bag was less.