Under the standalone product detail, maybe having two weights could work for getting more accurate shipping weights. For example: under general tab, you could have the actual product weight. This would be used for when multiple different products are combined into one order. Then there would also be a shipping weight under the shipping tab, that would account for any packing material and the box/mailer.
There could also be an optional area to add in the dimensions AND weights of all boxes and mailers that a person/company uses. As a result, when a box is selected (in a drop down menu), that weight info is automatically added as well.