The shipping cost of these will likely be based on "dimensional weight", which means that, because of their volume, you'll probably be charged according to size, not weight.
I learned about this after I shipped some packing foam to a buddy of mine via my FedEx account. I didn't bother to enter the package sizes into the shipping calculator. I put the shipping labels on the boxes and dropped them at my local FedEx. I though the shipping cost of the three boxes would be about $40. It cost me about $100. (At least it was good packing foam.)