Pretty much every additional service that USPS offers says that:
You can add the following options to… Insure the contents. Insurance for loss or damage up to $5,000. Get tracking information. Signature Confirmation™ to get added security by requiring a signature. Must meet additional requirements. Get proof of mailing. Certificate of Mailing as a way of proving the date you sent it. Must meet additional requirements. Get proof of delivery. Return Receipt with proof of delivery sent by email or postcard showing the recipient’s signature. Must meet additional requirements. Return Receipt for Merchandise with proof that you mailed the contents to your customer and proof of delivery sent by email or mailing receipt and postcard. Must meet additional requirements. Get only authorized recipients to sign for the delivery. Restricted Delivery to specify who is allowed to sign for the delivery. Get your customers to pay for merchandise. Collect on Delivery (COD) Recipient pays for merchandise and shipping when they receive the package. Must meet additional requirements. Send contents needing extra care. Special Handling to get preferential handling.
The additional requirements listed are: Purchase online or at your Post Office. Present COD pieces for mailing to a Postal employee.
Now I do not know whether or not USPS will actually ship MJ but your comment about COD is definitely wrong like so much else you post.