It's even more interesting than that with the current hybrid delivery services. Common carriers contract with the USPS for home delivery of packages and the USPS contracts with common carriers for long haul shipments of mail/parcels. What set of rules are they using?
If I, as a certified hazmat shipper, make a mistake, I, not my employer, am personally responsible for the error and can be personably fined by DOT or the FAA as well as the common carrier that accepted the package. The minimum fine is $750 for both the shipper and the carrier so the carrier has an incoming inspection of all hazmat packaging. If they find out that you made a mistake, they return the package to you under an exemption and you still have to pay the full shipping charges even though they did not deliver them. I stopped using UPS because I had to print out the shipping labels labels on their shipping forms using their software package and it printed out some of the information in 4 point type which they said was illegible but there was no obvious way to change the fort size. As a result, I ship all my hazmat via YRC because their software generates .pdf forms that always prints properly and gives me a hardcopy of everything generated for my records.
For the record, when you make up a common carrier hazmat motor shipment, you must include a hard copy paperwork of the (M)SDS, the emergency response plan, a copy of any special permit and authorization to ship rocket motors, and two signed copies of the shipping paperwork in an external document pouch, and affix the certified hazmat shipping box the proper descriptive contents label, a Class 4.1 or Class 1 diamond hazmat labels, special permit number label, and this side up arrows, and pack all the motors in the appropriate inner containers. When it gets picked up, the driver signs the paperwork and leaves you with a copy for your records which you are required to keep for 2 years just in case there is a problem or a DOT inspection. It's a PITA.
That's why it's not practical for an individual to ship rocket motors by common carrier. It's going to cost you several thousand dollars to get hazmat certified and to set up a commercial account with a commercial carrier, and then you have to purchase the DOT hazmat shipping boxes, labels, etc, and get the DOT required special permits (which takes a few months). If you go to this trouble, and ship rocket motors in a plain box without declaring hazmat, you are looking at thousands in fines if you get caught, so it's not worth the risk. Mailing small motors via USPS is much simpler however it still will take a couple months to get USPS written authorization to do it.
Bottom line is that it's quicker, cheaper and simpler to sell the motors you don't use locally before you go home.
Bob