it depends on the shipper and the person declaring what they are shipping.

technically, shipping compressed non flammable gas is considered hazardous product by most major carriers. you have to pay extra $$$ to ship it.
most people who ship it just wrap it up good to prevent damages to cylinder and ship non declared with no problems.
problem is IF it gets crushed and releases the product you will get charged a LOT, i mean a LOT LOT of $$$.

use you best judgment.