The app generates trackable links that you can send through your preferred text messaging app on your phone. Users are not sending trackable links through the app itself; the app only generates these links and then generates a pre-written text message that the user then sends through their text messaging app to others.
This means if you're texting trackable links internationally, unless you have a plan that includes international texting, you will be charged additional fees for text messages exchanged with international users. Make sure to check with your cell phone provider first to see if your plan includes international text messaging before sending a trackable link to someone in a different country.
If you want to avoid extra charges, try using a different text messaging app like Skype or WhatsApp to send trackable links internationally. Additionally, you can always generate a trackable link, copy this link, then paste it into an email or Facebook message to avoid extra charges.