When items are inventoried, they are weighed individually and their weights are input into my records. When the customer requests for the shipping fee quote, the weights of these items are added up, and then multiplied by 110% to include the weight of packaging materials. We assume that weight of the packaging materials is 10% that of the weight of the items. It is usually quite an accurate estimation because for heavier packages, more and sturdier packaging material is required for packaging.
Usually the difference in the estimated weight and weight measured by the post office / courier company is not significant to warrant a different shipping fee value. If it is, you would be informed and given a refund for the difference.