When is a message charged? I opened my contacts, clicked my friend's name and using his email, sent him a message. My friend's email is associated with an apple account. Yet when the message was delivered, it turned green. Does it mean that I was charged for the message? Is it necessary that the sender is charged whenever the message turns green? I don't have an SMS package or any other prepaid options. I have a simple AT&amp;T plan with limited talk time and 300 MB of data.

My friend lives in a different country and uses an iPod through his home wifi.

P.S.: I have asked this question on the official Apple forum (link). I think SE is a better place for such questions.
When the text bubble is green instead of blue, you're sending a SMS instead of using iMessage, being, therefore, charged for that. 

Messages are sent as SMS when iMessage is unavailable, unless you disable this under Settings / Messages / Send as SMS.

So, as I understand it, you either sent it to an non Apple ID email address, or the text couldn't reach the receiver through iMessage, changing to SMS mode.