I think the problem is the ratio between the size of the box and the weight. If you can get more weight in there it might be more cost effective. You not only get charged for the weight, but also for the box size. So if you "under pack" a larger box, you get bumped up to a higher price anyway. Maybe someone else can explain a little better than I can, or you can email the CFCS folks and ask them about it.
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