If you have noticed that you are running out of credits fairly quickly, this could possibly mean that you may benefit from using a higher plan. The bigger the plan is, the cheaper your messages become. To determine the best plan, you may want to review the difference between MMS messages and SMS messages.
To view more plan options, click on the credits drawer on the bottom left of the blue panel. Then, click "Change Plan".
Once the plan options appear, you will see what is included in each plan. The rule of thumb for choosing a plan is to base it off of the amount of contacts that will be receiving messages from you or one credit per contact. Please note that MMS messages may cost more depending on your plan.