Sometimes there can be issues with the App Store when attempting to purchase the Interchange Pro upgrade. These issues are beyond our control, but we can suggest some things to try in order to get around any issues.
Firstly, restart your iPad. This clears a lot of Apple in-app purchase issues.
Next, check the Apple ID that you've entered for payments. Go to Settings->Apple ID (the top area)-> iTunes and App Store and check that the Apple ID in there is as expected.
Now go back to Interchange, and tap Go Pro. Don't worry - you won't get charged again. Apple will see that your Apple ID has been used to pay for Interchange before and it will just restore the previous purchase.