Per-message event handling
After the basket has received the command to add a product in it, the web application will not yet be updated with the new basket information.
In terms of responsibility, it is correct that a command handler does not update the UI. Instead it has the responsibility of notifying that a product was added into the basket, and that the total and the content have changed.
This is an important concept for evolutional software. Events are a great way of decoupling, not only on actual features, but also with future, hypothetical features. If in future there is a need for another feature depending on how the basket updated, then none of the existing code would be affected as just another worker would be necessary to generate another ...