When I did shipping and receiving we did exactly that. We’d make bales of cardboard in our baleing machine and stack the bales in shipping container and sent it back. They paid the store like $20 bucks per bale.
Where I lived at the time all shipping containers came in and went out on the barge so filling them up vs having them empty was a negligible cost difference according to the shipping company.
I’ve played enough D&D to know a Dragon hiding themselves with Illusion magic when I see one. Just leave your gold as tribute and don’t look’em in the eye.