Not sure it it's too late to help, but if you want to use two interlocking halves they need to be separated by a few mms in the file so they don't accidentally fuse together during printing. (There is a small minimum "clearance" distance quoted in the Guidelines, but I like to err on the cautious side and move the items far enough apart to see clear "daylight" between them on the CAD screen...)
Either sprue up the halves, trap them in a cage, or tie them together on a keyring-style loop through a convenient hole so each printed "unit" has all the parts for one USB case - that way you save the mental health of the poor tech that has to handle/clean/pack them, and don't need to waste time matching up parts when the kids are getting restless.
One thing to keep in mind is how the parts will connect - the printed parts are immune to solvent glues. You can use superglue (always fun when you need to supervise others...) or design in some kind of mechanical tab-based fixtures to secure them.
There's no discount for multiple copies. Just looking at a USB stick here, I'm guessing it is about 1 or 1.5 cubic cms of material (assuming a 1mm-thick skin), so in FD at $2.39/cm3 that's under $4 each. Much more expensive than injected plastic, but not much set-up cost for limited runs. (IM plastics tooling runs in the 10s of thousands of dollars, before a single item is ready for sale...)
Hope this helps!