Hi cemc91,
If you are responding to a text that was an email address that is included in your standard texting plan. If you are texting a US based number, no matter where they are in the world its part of your standard plan.
If you text anyone that doesn't have a US based cell number the charge is .25 cents a text.
If you are on a Mobile Share Value plan, you have unlimited international text messaging to 190 countries around the world (Japan being one of them) so you would have no charges
Hope this helps!