UPack is the one I'm planning on going with for my move.
I don't have that much, but it's enough to where I can't ship it all via USPS or UPS.
If you only have enough to fill 12' of space, that's all you're charged for, and they don't lose anything, because they fill the trailer up with stuff from another family, or freight that needs to go in the direction your stuff is going.
The logistical planning division for that company must be vey interesting.