To cancel your plan, you will need to uninstall the app
You can follow these steps to uninstall the app on your end:
Go to your Shopify dashboard.
Click "Settings".
Click "Apps and Sales Channels", locate the Sign Customiser app and click Uninstall.
After removing the app, you will unsubscribe to the app’s plan automatically. You can find a more detailed guide on how to uninstall a Shopify app on this page.