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function wc_shipwire_set_custom_shipping_method( $fields, $order_id ) {

    $fields['shipping_code'] = 'GD';

    $order = wc_get_order( $order_id );

    foreach ( $order->get_shipping_methods() as $method ) {
        if ( '2-Day Shipping' == $method['name'] ) {
                $fields['shipping_code'] = '2D';
                break;
        }
    }

    return $fields;
}
add_filter( 'wc_shipwire_order_defaults', 'wc_shipwire_set_custom_shipping_method', 10, 2 );