Add a new list item template

Create a new FlexForm select box entry by Page-TypoScript:

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

Create a new Fluid-Partial in your Template path:

Partials/Product/List/Item/MyListItem.html

Attention: The FlexForm item index must be equal to Template filename!