At any time you can add additional messaging credits to your account, and pay the same rate per segment as your current messaging plan.
If your account balance hits 0, you will be unable to send additional messages. Your current account balance is visible on your by navigating to the Settings > Plan & Billing > General page.
To add account credit:
- In the left navigation select Settings > Plan & Billing
- Select the General tab
- Click the Add More Credits button and select your desired credit amount