I'm having troubles with the purchase of your mobile App

The mobile App is not sold directly by GSi, you're probably buying it from the Google Play Store or Apple's App Store, according to your mobile device. In this case, for everything concerning the payment, the download or the installation of a paid App, you should contact the appropriate support center:

Google Play Support:
https://support.google.com/googleplay/

App Store Support:
https://support.apple.com/apps