Appointfix
Go to Menu → Settings → Account, and tap “Buy more text messages.”
Select the pack you need (50 messages – $2.99; 100 messages – $5.99 ; 250 messages – $14.99; 500 messages – $29.99).
The prices are displayed in your currency in the app.
Unused extra messages are carried over to the following usage cycle, after which they will expire.