This really is a no-brainer, take your licks don't try to fight it, at this point best you can do is say "I made a major mistake, and I'm really sorry" and hope they keep you.
This really is something that doesn't need to be a written rule because just like:
Selling drugs(or anything else) out of your truck
Paint a swastika on your face <-there is no rule about this, but you wouldn't do it right?!
etc etc
As an aside I was a helper many moons ago at UPS, and my driver would switch with the local FX driver on occasion... unfortunately times have changed, and nearly real-time tracking cannot be avoided he retired a happy rich man