We are sorry that you are not satisfied with the app. You can refund via the Google Play site, please read their support page. You also can send a request to firstname.lastname@example.org. We do refund for a purchase made no more than 30 days before the request date. Please send us your purchase id (a string like GPA.123-456-78910). To find it, look at your purchase history.
First of all, be sure that your device is linked to the same Google account that was used for the purchase. If you are sure, it looks like there is an issue with the Google Play app on your device: it did not get the list of purchases correctly. Typically, the issue goes away in a few hours. But if you want, you can fix it manually. There are two ways.
You do your purchase on Google Play, and we have no additional information about the purchase. We even cannot check, if an account purchased the Premium or not. You can review your purchase history on the Google site.