It's pressurised vessels. If sending anything like that it's going to be difficult. Some say only safe way is by an RFD. they will have a different account to the public but even then they may have difficulty in sending some items. I tried to get an RFD to send a dive tank for me. Only one could do it. The same carrier that the dive bottle company use may not take a dive bottle from an RFD as it's a different account that the dive bottle company use.
From what I have seen on here .you send it as "ground delivery only.no flying" but a pressure vessel ? I'm not sure.
You will only get people "own views" on here but some on here know the score. Angry bear and logunner are the two I trust among a few others to have the right advice on here. One being an ex royal mail worker that helped people "find" lost items for members on here.