Update: If you are experiencing this error or wish to use a credit card to purchase ZipGrade, please follow these instructions to use the website to extend your ZipGrade subscription.
We've seen a few users have trouble purchasing ZipGrade due to error coming from Google Play. The most common error code is:
This error may appear for a number of reasons, only some of which are under the user's control. Among other reasons, this error will appear if there is a version mis-match in data stored on the Google account and the Google Play version. Here are the generally recommended steps to take if you encounter this error:
1. On your device, go to 'Settings'
2. Find 'Applications' or 'Apps' (depends on OS version)
3. Find and select 'Google Play Store'. Press to Clear Data and/or Clear Cache.
4. Return to 'Settings' and find 'Accounts'
5. Find 'Google' and select to remove this account. (If you have more than one Google Account, remove them all).
6. Reboot your device. (Hold down power button until you see 'power down'.)
7. Start 'Google Play Store' and follow prompts to login with your account.
8. Go to ZipGrade and attempt to purchase the in-app purchase again.
Please let me (firstname.lastname@example.org) if this works for you or not. We're continuing to actively track this issue.
Unfortunately, this error is internal to Google's code. If you continue to encounter this error, you may wish to contact Google Play support via the Google Play support website: https://support.google.com/googleplay