Add template for list item

First add a new entry in the FlexForm for the selectbox:

TCEFORM.tt_content.pi_flexform.shop_products.sDEF {
    settings\.displayType {
        addItems {
            MyListItem = My list item label
        }
    }
}

Then create the appropriate template file:

Partials/Product/List/Item/MyListItem.html

Tip: The item index in the FlexForm configuration must match the template file name!