You can purchase Audiomack premium via the app.
Here are the steps to follow:
1. In the app, go to My Library
2. Tap on the settings icon (top left)
3. Tap on the button 'Why Upgrade?'
4. Follow the steps to upgrade to premium via the app store.
**Audiomack does not have the the access nor the authority to resolve issues regarding payment info i.e. card or banking. If you're having trouble with a payment option inside the app store, you have to resolve that with the app store.**