When I was selling on eBay, I used a kitchen scale for larger items and a weight watchers scale for the smaller items. To test the accuracy of the scales, occasionally I'd set a full box or bag of sugar on them and see if the weights were right.
Of course, everything was weighed in their shipping box along with the amount of packing material I'd anticipate using. This would get me as close as possible to the shipping weight.
So, here's the rub; on kitchen scales, the higher the weight the more "off" it will be. For things over 20 pounds, I'd add a few extra pounds to "make sure". For things between 25-50# weighed on the bathroom scale, I added 5 pounds just to be safe. I'd base the shipping cost by adding the weight plus the safety margin plus insurance.