How to turn Postcode field into Dropdown menu? (WooCommerce)

Excuse the sloppy explanation, I am a novice.

I am building a WooCommerce-based store. I have a list of postcodes, each one has a different shipping cost attached through Shipping Zones (some provide free shipping, some have a flat rate). When the customer goes to the checkout page, he needs to type his postcode number in the input field. Depending on postcode, an order preview will show different shipping total (free or flat rate). Here's how the input field looks like in class-wc-countries.php:

public function get_default_address_fields() {
    $fields = array(
    'postcode' => array(
            'label'        => __( 'Postcode/ZIP', 'woocommerce' ),
            'required'     => true,
            'class'        => array( 'form-row-first', 'address-field' ),
            'clear'        => true,
            'validate'     => array( 'postcode' ),
            'autocomplete' => 'postal-code',
        ),
    );

However, what I want to do is to turn this field into a dropdown menu, so the customer could just select his postcode option rather than type it. I managed to make it dropdown, but whenever I choose any option it doesn't seem to change shipping total as it would with input field. Here's what I did:

public function get_default_address_fields() {
    $fields = array(
        'postcode' => array(
            'label'        => __( 'Postcode/ZIP', 'woocommerce' ),
            'required'     => true,
            'class'        => array( 'form-row-first', 'address-field' ),
            'clear'        => true,
            'validate'     => array( 'postcode' ),
            'autocomplete' => 'postal-code',
            'type' => 'select',
            'options' => array(
                              'opt1' => "001122", "112200", "334400")
        ),
    );

Am I missing something? How do I make these dropdown options change shipping total?