Shoot us a PM with your details so we can check if sending international SMS is part of the plan.
In the event that your mobile plan does not include International SMS therefore it will be Pay-as-you Go.
SMS to International Numbers is 50¢ per message (max 160 characters). You will need to top up your account to be able to send SMS (outside Australia).
Here's an article on how you can add a Prepaid Balance:
I used to be able to send text messages to my son in the US, last successful about a year ago, and then it stopped working with a "failed to send" message. I can receive from him just fine. How would I fix this?