I ran into this late last summer. for some reason since a box containing a frame only is much lighter than a larger bix with a full bike it cost more. it has something to do with weight X the volume of the box.
I guess beacause the heavier boxes fill the truck to gross weight and the large light boxes would cause it to 'bulk out' before reaching gross weight
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“One morning you wake up, the girl is gone, the bikes are gone, all that's left behind is a pair of old tires and a tube of tubular glue, all squeezed out"
Sugar "Kane" Kowalczyk