Modify transaction data sent to authorize.net

// Originally from here http://pastebin.com/XceGeuBF
// Reference: http://www.gravityhelp.com/forums/topic/sending-additional-information-to-authorizenet-via-namevalue-pairs?replies=5#post-143109
/**
 * E-COMM -- Authorize.net API for GravityForms
 *
 * Modify the transaction data sent to Authorize.net
 * Ref: plugins/gravityformsauthorizenet/authorizenet.php
 */

add_filter('gform_authorizenet_before_single_payment','my_anet_custom_fields', 10, 2);
function my_anet_custom_fields($transaction, $form_data) {

    # GET DATA FROM FORM INPUT
    $form_data["my_invoice"] = rgpost("input_7"); // different for each form!
    $form_data["customer_id"] = rgpost("input_8"); // different for each form!
    $form_data["custom_data"] = rgpost("input_9"); // different for each form!

    # ADD DATA TO TRANSACTION OBJECT
    $transaction->invoice_num   = $form_data["my_invoice"];
    $transaction->cust_id       = $form_data["customer_id"];
    $transaction->description   = 'My Custom Data: ' . $form_data["custom_data"];

    # CUSTOM FIELDS: based on the AuthNet AIM api, this should work but it does NOT.
    //$transaction->setCustomField("Color", "Blue");

    return $transaction;
}