For my earlier post, I had only done a quick scan of this "Publication 52" (specifically, Appendix A) which DOES say that shipment of rocket motors is prohibited (p. 193).
Then when you go back and wade through the actual text (p. 16, and p. 26) the same publication says it is OK to ship a few motors "when prior written permission has been obtained from the Manager, Business Mail Acceptance, USPS Headquarters, Washington, D.C." From wayback, I remember there is a specific format you are supposed to use when requesting this USPS permission, including descriptions of exactly what you propose to do.
phaar, my point is simply: you better go talk to the post office people, and not ask for instructions from a forum like this. We don't have the authority to tell you how to ship. We probably don't even have the authority to determine what color blindfold you will get to use in front of the firing squad.