<?php
// Create a UK zone using the WooCommerce shipping zone API
function caz_create_shipping_zones(): void {
    // Check if zone already exists before creating
    $existing = WC_Shipping_Zones::get_zones();
    foreach ( $existing as $zone_data ) {
        if ( $zone_data['zone_name'] === 'United Kingdom' ) return;
    }

    $uk_zone = new WC_Shipping_Zone();
    $uk_zone->set_zone_name( 'United Kingdom' );
    $uk_zone->set_zone_order( 1 );

    // Add UK as a region using the WooCommerce shipping zone API
    $uk_zone->add_location( 'GB', 'country' );

    // Add Scottish Highlands postcodes as an exclusion zone
    $uk_zone->add_location( 'IV*', 'postcode' );  // prefix match

    $uk_zone->save();

    // Add flat rate method to the zone
    $uk_zone->add_shipping_method( 'flat_rate' );

    // Add free shipping method with minimum order condition
    $free_id = $uk_zone->add_shipping_method( 'free_shipping' );
    $methods = $uk_zone->get_shipping_methods( true );
    foreach ( $methods as $method ) {
        if ( $method->id === 'free_shipping' ) {
            $method->update_option( 'requires', 'min_amount' );
            $method->update_option( 'min_amount', '50' );
        }
    }
}

<?php
add_filter( 'woocommerce_shipping_methods', 'caz_register_shipping_method' );

function caz_register_shipping_method( array $methods ): array {
    $methods['caz_carrier_rates'] = CAZ_Carrier_Shipping::class;
    return $methods;
}

class CAZ_Carrier_Shipping extends WC_Shipping_Method {

    public function __construct( int $instance_id = 0 ) {
        $this->id                 = 'caz_carrier_rates';
        $this->instance_id        = absint( $instance_id );
        $this->method_title       = __( 'Carrier Calculated Rates', 'caz-ship' );
        $this->method_description = __( 'Dynamic rates from carrier API via WooCommerce shipping zone API.', 'caz-ship' );
        $this->supports           = [ 'shipping-zones', 'instance-settings' ];
        $this->title              = $this->get_option( 'title', 'Carrier Delivery' );
        $this->init();
    }

    public function init(): void {
        $this->init_form_fields();
        $this->init_settings();
        add_action( 'woocommerce_update_options_shipping_' . $this->id,
                    [ $this, 'process_admin_options' ] );
    }

    public function init_form_fields(): void {
        $this->instance_form_fields = [
            'title'       => [ 'title' => 'Method title', 'type' => 'text', 'default' => 'Carrier Delivery' ],
            'max_weight'  => [ 'title' => 'Max weight (kg)', 'type' => 'number', 'default' => '30' ],
        ];
    }

    // WooCommerce shipping zone API: calculate_shipping() is called at checkout
    public function calculate_shipping( array $package = [] ): void {
        $postcode    = $package['destination']['postcode'] ?? '';
        $cart_weight = WC()->cart->get_cart_contents_weight();
        $cart_total  = WC()->cart->get_cart_contents_total();

        // Dynamic rate logic — replace with real carrier API call
        $rate = $this->get_rate_for( $postcode, $cart_weight, $cart_total );

        if ( $rate !== null ) {
            $this->add_rate([
                'id'    => $this->get_rate_id(),
                'label' => $this->title,
                'cost'  => $rate,
            ]);
        }
    }

    private function get_rate_for( string $postcode, float $weight, float $total ): ?float {
        // Free for orders over £75
        if ( $total >= 75 ) return 0.00;
        // Higher rate for Scottish Highlands (IV, KW, PH, etc.)
        if ( preg_match( '/^(IV|KW|PH|PA|FK|KY)/i', $postcode ) ) return 12.95;
        // Standard weight-based rate
        return match ( true ) {
            $weight <= 1   => 3.99,
            $weight <= 5   => 5.99,
            $weight <= 20  => 9.99,
            default        => 15.99,
        };
    }
}