Add custom content to cart and checkout pages (only if a certain product ID is in the cart) with an action hook

conditional cart checkout hook by product ID

// CHECK FOR SPECIFIC PRODUCT ID IN CART
function madcow_custom_product_id_is_in_the_cart($ids) {
    // Products currently in the cart
    $cart_ids = array();
    // Find each product in the cart and add it to the $cart_ids array
    foreach (WC()->cart->get_cart() as $cart_item_key => $values) {
        $cart_product = $values['data'];
        $cart_ids[]   = $cart_product->id;
    }
    // If one of the special products are in the cart, return true.
    if (!empty(array_intersect($ids, $cart_ids))) {
        return true;
    } else {
        return false;
    }
}


// ADD BANNER TO CART PAGE BASED ON PRODUCT IDs IN CART
add_action('woocommerce_before_cart', 'madcow_above_cart_and_checkout_message');
add_action('woocommerce_after_order_notes', 'madcow_above_cart_and_checkout_message');
function madcow_above_cart_and_checkout_message() {
    $ids = array(20);
    if (madcow_custom_product_id_is_in_the_cart($ids)) :
        echo '<div class="top-cart-message"><h2>This is our cool message</h2></div>';
    endif;
}