incidentally new packages from this supplier are all properly labelled with exact contents,
a “Xn” warning (Harmful), what appears to be accurate diagrams of the molecule (I might photograph or scan it so any brainy people can see) and “NOT FOR HUMAN CONSUMPTION” as well as “KEEP OUT OF REACH OF CHILDREN”.

TBH that makes them no less illegal in the postal system than sending a pack of ferric chloride crystals or any other COSHH substances, I think as long as they are Xn, Xi (harmful) they can just about get through the post or otherwise many useful things for the workplace could not be sent mail order.