Just my opinion (based on my experience of buying several PCP Rifles and Pistols from Europe (and spare cylinders in separate shipments) : there doesn't seem to be a problem and I'm guessing that it's because some Courriers accept "empty pressure vessels" for shipment from Trade Account Holders because they have the necessary level of certification regarding H + S issues involed with their shipment (as does ParcelForce in the UK).
Vic Thompson.