well you could apply $700+ to your account but they wont just take out the $30/month out of it  instead your first few bills would be deducted from it so you would just need to pay the $700 and then like when you get your bill say if its $130 then your balance would go from a credit of $700 to $570 you would then need to pay $100 to bring it back up to $670.  If that all makes sense.