function wpf_save_custom_checkout_field( $order_id ) {

    if ( ! empty( $_POST['special-offers'] ) ) {
        update_post_meta( $order_id, 'special-offers', true );
    }

}

add_action( 'woocommerce_checkout_update_order_meta', 'wpf_save_custom_checkout_field' );

function get_product_data( $order_data, $order ) {

    $order_data['special-offers'] = get_post_meta( $order->get_id(), 'special-offers', true );

    if ( empty( $order_data['special-offers'] ) ) {
        $order_data['special-offers'] = null;
    }

    return $order_data;

}

add_filter( 'wpf_woocommerce_customer_data', 'get_product_data', 10, 2 );