According to the logistics guy at my job, UPS and FedEx will ship empty CNG tanks without hazmat, as long as it's ground (airplanes are a different story).
http://forums.pda.sailboatowners.com...2&pp=20&page=2 seems to have similar information.
We do have a fill station here in Madison, but not one near my boat (Apostle Islands), and my one tank only lasted 1/2 the season last year.
I'll call UPS/FedEx and ask.
Thanks!!!