How can I add a “remove all cart items” buttons ?
There’s not currently such a feature built in, so I’ve logged a feature request.
There’s a $order->forgetOrderId() method that will make the browser forget the order it was using (essentially resetting it entirely - although it may still exist in the database) which is probably the most thorough reset. Alternatively iterating over the items and calling $order->removeItem($item) for each one would just remove items while keeping addresses and such.