In some rare cases, an error might pop up when fetching the in-app purchase prices from the App Store server. Please disconnect from Wi-Fi 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 support@stafftraveler.com.
Alternatively, use StaffTraveler for web at https://stafftraveler.app to purchase credits using your credit card.