When is a message charged?
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.