You don't have to ship anything in particular... in fact, you could just send an empty crate so that it is easy to ship your robot back home after the competition.
But that means you only get to bring 65 pounds of fabricated parts in to the competition with you. Keep in mind that if a motor is mounted to a transmission, or if it has a terminal attached to the wires that it is now a fabricated part. You also get to bring your control board (the classmate, joysticks, etc. that never goes on the playing field) and that doesn't count towards the 65 pounds.
So, if your entire robot, including both sets of bumpers (but not battery) weighs less than 65 pounds, you don't have to ship it. You might, however, want to ship any fabricated spare or replacement parts that you think you might need. You might even want to ship 50 pounds of ballast so that you can add it to your robot to help keep the 120 pound robots from pushing you all over the field.
You might also want to check out this thread
http://www.chiefdelphi.com/forums/sh...ght=witholding
I'd say that if you've got your bumpers made, that you should ship them, along with any spare parts that you think you might need that are no longer EXACTLY as they were when you purchased them. That way you can maximize your use of the 65 pound witholding limit.
The rule is there for you to use... so study it and use it wisely.
Jason