FR: Thats what I was wondering. If I show up at UPS, and they ask , " Whats in the cardboard box", and I say an old outboard gas tank. Whats going to be the reaction. In this day, and age, probably a SWAT team, and pick-up load of dogs.
I don't think the post office will ship hazordous items. I would call UPS or fedEx or both and ask them. By calling both you can shop around for ther best rate.