That's just a general message that's sent. The terminology of "overages" is subjective here. You don't have any "overages" because you don't have an allowance, however your per message cost is higher than paying for a package. With 51 messages used your total right now would be $10.20 and will continue to increase as your cycle continues. However, the lowest text package you can add is only $5 and allows 250 messages.
Adding that package is something you may want to look into if you're using more than 25 messages a month (25 x .20)=$5. Adding the package would help your bill be more consistant if you continue to use messaging at the same rate you are now.