It can be difficult to estimate an accurate shipping weight of the artwork but you can provide an estimated approximate weight. You can do this by providing approximate weight of the artwork along with weight of the packaging material like the crates, foam board etc. If you are still not sure of the weight, you can provide an approximate weight as per your best judgement. It is important that you provide the best accurate weight for minimizing any problems associated during the shipping of the parcel. It is advisable to add additional physical weight to your artwork packaging in order to avoid additional charges on shipping.