To purchase credits, an administrative user can go to Settings>Setup>General Settings> SMS and Voice. Click the pencil icon to open the following window.

From this window you can view the number of credits on your account and purchase more by clicking "Buy More SMS and Voice Credits." This will open a new window.

Here, you can calculate your total purchase and select your payment method- using your autopay information on file, a credit card, or an e-check. Notice a minimum purchase of 100 credits is required.

Click "Finish" when you are done entering your payment information to process the purchase. Your purchased credits will now be available for use.