You can only subscribe to the Paramount+ with SHOWTIME bundle through the Paramount+ website or app. If you subscribe through the Paramount+ website, you will be billed directly by Paramount+. If you subscribe through the Paramount+ app on your device, you will be billed by the applicable billing provider for your device (Apple, Amazon, Google Play, Roku).
If you are currently a SHOWTIME subscriber without the bundle and you then subscribe to the bundle through Paramount+, you should cancel your existing SHOWTIME subscription to avoid being charged for multiple SHOWTIME subscriptions.