Within all plans, credits are used for sending SMS messages and for exporting contacts to MailChimp. These credits can be purchased in your account once you have insufficient credits. Purchase your credits via menu option Administration -> Credits. We will send you an invoice (Windows) or transaction is handled via Mac App Store.
Credits cost $5.99 for 100 credits, $29.99 for 500 credits and $54.99 for 1.000 credits.