If you are overseas, I think it's better to access Starbucks on your desktop, with a VPN, and check on iplocation . net if it's really showing you in the US.
I didn't have any problem using the app, but after two transactions, my account was blocked from reloading the Starbucks card (my account got flagged as suspicious, probably related to my location). After a few inquiries with Starbucks (they have a very good support team), they unflagged my account and it's working again, but now I'll only do reloads on my desktop, checking beforehand if I'm in the "US".