I would let the air out and put it in a good quality cardboard box surrounded by some soft packing material like foam peanut, styrofoam, or even newspaper that is bunched up ensure that the neck is well protected so it won’t be damaged.
Empty/double boxed and well padded/secured. Last one I purchased was wrapped in a couple layers of other cardboard boxes within a properly sized outer box.If you are shipping it with the valve/fill assembly, especially if still attached to the tank, make sure it's extra well protected.