With the flat rate box its doesn't go by weight , so you stuff the box supposedly. A little known fact is that if the box weighs less then 4 lbs you can ship it out as a letter which saves a ton even if it is a box. Also when we ship out international we have found that to ship out priority without regard to box size is cheaper then if we put a box size in our manifest. The company I work for ships worldwide and we have never had a box returned for more postage
Sharon