Arggh, I should have seen that before. Do you have a customized /includes/modules/YOUR_TEMPLATE/attributes.php? For a 'out-of-the-box' Zen Cart attributes.php, the 'id' value will come back as an array in options_id => options_values_id format, like the following:
Code:
[14-Dec-2014 14:56:50 America/Los_Angeles] /home/XXXXXXXX/public_html/ZEN153/includes/extra_cart_actions/restrict_digital_downloads.php:Array
(
    [securityToken] => Not sure if it is OK to post that
    [cart_quantity] => 1
    [products_id] => 776
    [id] => Array
        (
            [6] => 61
 
        )

)