Is it possible to set up the subscription so it expires once my time runs out, and if I wish to have more time I can purchase it again? I'm just saying i'd prefer it to not automatically bill my card.

Use PayByCash and the PayPal option with a credit card. It's roundabout, but it should work.

Automatically billing for large denominations of time should bring up a message whenever you login about 2 weeks before time runs out asking you to verify if you want to purchase the same amount.

Simply hit unsubscribe and you will keep the time you have and your card will not be billed until you subscribe again. Granted you have to put your info in again but you will not be automatically billed.