When you have something like a down comforter that is not only bulky but heavy you need to find a box first of all, then you need to try to get as much air out of the item as possible by squeezing it out.  Put it in your box while getting as much air out as you can and then taping the box to make it as small as possible. The item will ship for much less if you make it as small as it can be.  It could save you as much as $20 on just one item.