SnapChating someone overseas

Hi there. I am really careful about incurring any fees and im a new user when it comes to smartphones. My question is, will I incur any additional fees from ATT if i send/receive snapchats on my phone to/from somebody that is overseas? Either when I use my data plan or my home WiFi?

I don't want to receive an astronomical bill in the future and I already made sure that SnapChat ( the makers of the app) does not charge for any international use.

To reiterate, I am in the U.S. but my friend is the one that is overseas.