In some rare cases, an error might pop up when fetching the in-app purchase prices from the App Store server. Please disconnect from WiFi and try again on your cellular connection. You might have to close the StaffTraveler app and relaunch it to make this work.
If the error keeps occurring, please send a screenshot of the error to [email protected].
Alternatively, use StaffTraveler for web at https://stafftraveler.app to purchase credits using your credit card.