Of course there are rules . . . but from your question, it is unclear as to whether you mean rules for competing in a postal or for hosting.
As for the rules of competition, here is the link to the "Championships" page for this year which includes the postal championships that have happened or will happen:
www.usms.org/.../
when you click the links to the pending competitions, you'll find the required entry forms with rules for participants.
As for hosting, I'm confident sure that there are rules available, but I don't know where they are.