Disabling it is really just a design decision so that customers don't see the N/A message and still click on it. I decided instead to just trick the forms API during the validation process to make that field not required for validation purposes, and it appears to work great. I added these lines after line 242 and the corresponding bit for the billing pane:
<?php
if (isset($_POST['panes']) && count($contents['delivery_zone']['#options']) == 1) {
$contents['delivery_zone']['#required'] = FALSE;
}
?>