For the MANDATORY monthly play time fees, you have 3 options:
More options than that, you forgot about the game-time cards. There are 2 ways you can get them:
From a physical store and input the code from the back into your WoW account on battle.net
From the online Blizzard store, then input the code from the resulting e-mail into your account on battle.net.
The first one is used primarily by people whose only payment methods are cash or check.
The second one is primarily used by people who wish to give gametime to a friend or family member (buy the time, forward the e-mail with the code to the intended recipient).
Both are also used by people who want to buy the gametime, but won't be able to play anytime soon. So they buy the gametime when they have the coin to do so, then save up the cards/codes for a later date when they'll have the time to actually play.