I just sent out the tank on my '77 Cordoba to be redone. Dump some liquid dish detergent(like Dawn)in the tank and squirt it with a good stream of water. Slosh it around and drain it. May have to do more than once. Let it dry and then wrap it in plastic before putting it in the box. Should be good to go. I shipped mine through Fedex.
I bought one from a dismantler in TX and had it shipped to PA several years ago. It arrived on a pallet with zero questions or problems that I was aware of.
Hazmat rules I think would dictate the tank be purged in order to ship it without making a hazmat declaration. That's how it works with fuel trailers. Even an empty trailer has to have a hazmat last contained manifest unless it has been steamed/purged.
Big fines for knowingly shipping without declaring.
as far as I am concerned it is no longer a gas tank once it is flushed out with water and no longer containing fuel. it now becomes a used water tank. ship it as that. problem solved. no hazards no problems.