Auto Complete for product variations use
You will need to write your own module and implement a hook_form_alter to override the normal Commerce way of displaying attribute selection.

A good example to look at is the Commerce Fancy Attributes module, which replaces the drop down with image selection. http://drupal.org/project/commerce_fancy_attributes

You can change the input type to autocomplete and point it to the correct callback url which returns your zipcode options in the format required for the product field.

$form['attribute_field_name']['#type'] = 'textfield';
$form['attribute_field_name']['#autocomplete_path'] = 'path/that/returns/list/of/zip/codes';