You can purchase additional SMS credits on whichever plan you are on, both free and paid alike.
To do so, head on to your dashboard from your client area menu.
Locate the “Account Overview” section towards the top left side of the dashboard widgets, and click on the number of “SMS Credits”.
A modal pop-up window will open. Input the number of how many SMS Credits you’d like to purchase, and then click on the “Place Order” button.
An invoice will be generated for you. Once paid, the extra SMS Credits are automatically allocated to your account.