The expiry of 1 year isn't for the refill cards, it's for the "phone number" for the hot spot itself. You can (as I did) let the hotspot sit inactive (with no monthly $$ put in) over the period you are not in the US and just buy a card when you return the next winter season as long as it isn't over a year.
I tried to find this detail in my documentation but couldn't. I would call AT&T to confirm. I called them 866-707-5550 (that's the activation number for the hotspot) to get the info.
On edit, I found the info on the website about halfway down the page on the right:
link
