Just to be clear, it can be sent by UPS and FedEx, they just charge through the nose for the hazardous materials tag.
This memo makes it extraordinarily clear that the shipment I received was actually illegal. It came China Post / USPS. Thank you.
When I read the original question I read "the post" as being any major delivery service. Reading it as the USPS only (when in the United States) is probably the more proper way to read that word.