On iOS (Apple):
On iOS devices all payments go through the Apple iTunes Store.
Please contact the Apple Support by following this link.
If you want to review your transaction history you can do it here.
On Android (Google):
On Android devices all payments go through Google Play.
Please contact our support team at email@example.com and add a proof of payment such as your payment history on Google to the mail.
You can review your order history by following this link.