PHP Notice: get_shipping_postcode was called incorrectly.

Hi,

I’m trying to add a shipping cost surcharge for particular suburbs. I’m using the following code, and functionally it works perfectly.

Unfortunately, it’s throwing the following PHP Notice:

get_shipping_postcode was called incorrectly. Customer properties should not be accessed directly.

add_action( 'woocommerce_cart_calculate_fees', 'outer_metro_add_cart_fee' );
 
function outer_metro_add_cart_fee() {  

$outermetro = array('5250','5251','5114');
    if( in_array( WC()->customer->get_shipping_postcode, $outermetro ) ) {
        $surcharge = 3; 
        WC()->cart->add_fee( __('Outer Metro Delivery', 'woocommerce'), $surcharge );
    }

}

Can anyone explain how I can get the shipping postcode from the checkout page without accessing it directly?

Thanks!