You can setup your account to automatically recharge to make sure you have enough in your balance. 

Navigate to the Billing tab (Profile > Settings > Billing tab)

You will need to have a credit card setup in the system and positive balance in your account to begin. Then click the Automatic Recharge button.

1. Enter an amount in the first field (Minimum of $50.00). If your credit balance is currently below the specified minimum then a credit purchase will occur when the recharge configuration is updated.

2. Enter the amount you wish to buy when recharging your balance.

3. Select the payment method that will be charged (make sure that you update this before the card expires)