I'm glad I use game cards. Helps a lot with the broken billing department that is blizzard.

If you don't want to buy 2 months at a time, you can buy one month codes directly from the blizzard store, in any quantity. I don't like keeping things on auto bill, Often I stop using something, but keep paying for it for a month or two before I remember to cancel the bill. This saves me a lot of hassle.