The flash sms message is a message that is immediately displayed on recipient phone and cannot be saved.
The recipient is able to reply only once to the sms message, then the sms message disapears.
The reply and the receiving of the reply is free of charge, however sending cost credits.
This is pay as you go service so, you need to buy credits.
On install you will receive free credits to test the app if you share the app you will receive more free credits.
Your phone bill will not be charged.