We need to try the following to *force* the correct answer from Google Billing.
Firstly try these steps:
1) General Settings > Apps > Google Play Store [UNINSTALL UPDATES].
2) Kill Textra and restart it.
If that does not work try these steps:
1) General Settings > Apps > 'Google Play Store', FORCE STOP and CLEAR DATA then open the Google Play app and press Accept.
2) Also check if you have multiple Google Play accounts, see which one you purchased under (as you must be logged into that account in Google Play or purchase on multiple accounts, that's just a google thing not us!). If you have multiple accounts, consider removing ones you no longer need. Ensure you are logged into Google Play with the account you purchased under.
3) Uninstall Textra, ***reboot*** and Install Textra from Google Play.
4) Startup Textra and it'll show you as a Pro user in Settings.