There's a FAQ on this inside the Support pages of the site. I'll try to get to it to post a link for you.
It essentially says that nothing untoward will happen to your account or character if you let the subscription lapse -- you just wouldn't be able to play.
Regarding not entering subscription info yet because you want to use a pre-paid card, that won't be a problem, either. ZOS confirmed on the main announcement that no one would lose a preorder bonus for a delayed subscription enrollment (I also got a response from ZOS directly to the same effect).