I am a drupal commerce beginner that has taken over a drupal commerce site for a client. The shop has a number of products that can be placed in a shopping cart and then there is a checkout process and all products are handled as regular stock items.

Now the client wants some items to be on order/non stock items.
If the customer puts one of these items in the shopping cart they should get a message saying that this is such an item and that normal delivery times don´t apply. My client alternatively/also wants this to be noted in some way in the checkout so that the customer has the option to deselect these types of products. I have no idea where to start with this. Any tips?