I don't know the exact weight of the Artwork I want to sell. Why should I mention this parameter?

I don't know the exact weight of the Artwork I want to sell. Why should I mention this parameter?

It is really important to note the correct weight of the artwork in order for us to calculate the shipping costs correctly. Please indicate the weight and dimensions of the artwork without wrapping material. During the upload of Artwork please clearly indicate if the Artwork will be delivered flat or rolled in a tube.