Using auto-updating country/zone select boxes  Ãœbercart has
been wired to automatically update a zone select box on a form
when the corresponding country select box is changed. In order
for this to work, you need to keep in mind two simple things:
1. You should use a similar prefix in the keys of the form array
for the country/zone select boxes. The JS that gets added uses
this to detect which form element should be updated when a
country select is changed. Example:
['ship_to']['delivery_zone'] =
uc_zone_select(uc_get_field_name('zone'));
$form['ship_to']['delivery_country'] =
uc_country_select(uc_get_field_name('country'));      Notice how
the keys both start with delivery_ and end with zone and country
respectively.    2. You have to include a special check in your
form function for the updated zones to be considered as valid
options for the select box. The Drupal forms API will only
accept a value for a select box that is included in the options
array when the form is built. Because we're dynam