Printing custom fields in invoice template (D7)

How can I get field values to show in the customer invoice template? I tried adding this line to my template for my "delivery date" custom field:

<?php print t('Delivery date:'); ?> <?php print $ucxf_delivery_date; ?>

but the field value doesn't show up in the invoice email.

There seems to be a fix in the D6 version but can find nothing here for D7.