I've seen and heard that people have been missing parts for their EB. I myself encountered this too.
I suggest politely trying to reach out to Mark and see what he can do for you.
Obviously I only recommend doing this is its a substantial part. If its just a crimp or nut or single wire or something small, suck it up, and visit the hardware store. You'll have the much more immediate satisfaction of finishing your bot then waiting on the USPS.
Just passing my $0.02 along. Mark from EB did a great job of resolving my issue. I can't guarantee you'll have the same experience, but I'd like to think that he'd at least try to assist you too where possible.