This article describes how to buy a module using ISPmanager. Virusdie is using as the example.
To purchase a service go to "Integration" ⇾ "Modules", find the module you need and click "Buy".
Then enter your username and password from your personal account and press "Ok". If you've entered that earlier, when buying another service, you'll be asked for a password only.
After that you will be redirected to the cart. Click "Buy".
Choose a preferred payment method and press "Pay" button.
Then you'll see the table with "Service is ready to use" message.