The app starts billing as soon as you accept the pricing on your Shopify store. In case you want to stop the billing, you can uninstall the app and you won't be charged anymore. If you forget to do this, unfortunately we wouldn't be able to help as the billing wasn't stopped on your end. If you believe the app charged you incorrectly, please reach out the support with the following details:
Your myshopify domain.
The billing cycle.
The reason why you believe the billing was incorrect.
And finally, the amount of refund you are requesting for.
To get your invoice/billing cycle, go to Shopify Settings > Billing and then look for the Pre-Order Now app.