WooCommerce将费用添加到woocommerce_cart_item_subtotal

Question

I am trying to add a fee I have set up using Gravity Forms Product Add Ons. The fee comes through in the product variations, but I'm having issues trying to:

1. Pull in within a filter
2. Adding the fee only once to the subtotal in the cart row.

Here is what I have so far:

// define the woocommerce_cart_item_subtotal callback
function filter_woocommerce_cart_item_subtotal(
        $product, $cart_item, $cart_item_key) {
    global $woocommerce;

    // 1. Find out if Setup fee is added
    $custom_setup_fee = get_post_meta($product->ID, 'value', true);
    echo $custom_setup_fee;

    /*
    echo WC()->cart->get_product_subtotal(
        $_product, $cart_item['quantity'] ), $cart_item, $cart_item_key;
    echo WC()->cart->get_product_subtotal(
        $_product, $cart_item['quantity'] );
    */

    // 2. If setup fee added, add to existing value

    // 3. If no setup fee just return value
    return $product;
};

// add the filter
add_filter(
    'woocommerce_cart_item_subtotal', 
    'filter_woocommerce_cart_item_subtotal', 10, 3);

The first variable called $custom_setup_fee does not pull in anything. Any ideas?