Postegro Charged More Than the Advertised Price

I purchased the Postegro app through the Apple App Store. Although the app's official website listed the price as $6, my account was charged $21 instead. Immediately after the payment was completed, I tried to open the app, but it continuously displayed a message saying "Your data is being updated" and never finished loading. As a result, I have been completely unable to access or use the app.

From the moment the payment was processed, I have not been able to use the application or receive any of the service I paid for. The order and invoice number for this purchase are available in my Apple account, and I can provide them if needed.

I request that the app be fixed so it opens and functions properly. I also ask for a clear explanation as to why I was charged $21 when the website advertised a price of $6. If I was overcharged, I request a refund of the excess amount as soon as possible.