You can purchase add-ons by going to "Account settings" and selecting the "Get Add-ons" option. Add-ons will allow you to expand the messaging allowance provided by your current plan for an additional payment. After selecting and purchasing an add-on, the information about it will be appended to your plan details, and the message count will be updated accordingly.
Add-ons will be active until the plan they are added onto expires. Only an account administrator can purchase add-ons. Every time you buy an add-on, you will be charged a single instant payment. Once your plan expires and another plan starts, the new plan will not have any of your previous add-ons. Only specific add-ons are applicable to a Free plan!