not personal experience, but reliable people on this forum have told of how fedex or whatever shipping within the states has a big big jump of price once a box is past a certain dimension.
you'll want to inform yourself of these details if thats a route you may go down.
For the edification of others…You’re correct. I played around on Bikeflights’ website. Bikeflights uses UPS. I got quotes using a certain box size at weights between 40 lbs. and 70 lbs. Price was always the same. Then I got quotes for a larger box size. Can’t remember the dimensions, but the price went up. Up to a certain point, size in the primary price driving factor.
In any event, the OP has figured it all out so I am definitely out now.