I have 2 A body LCAs without the pivot pins/shafts in a large top load flat rate box & the just fit. Box bulges a little, but I'll use a liberal amount of packing tape if I ship them.
just an FYI, the USPS gets crabby when you overstuff flat rate boxes. so don't be surprised if it shows back up on your doorstep with a bunch of red stamps, goes MIA in transit or gets to the destination beat to hell.
