将自定义args添加到Paypal Woocommerce

I need to add custom args to PayPal paying link.

The PHP function looks like this:

$paypal_args = apply_filters( 'woocommerce_paypal_args', $paypal_args );
add_filter( 'woocommerce_paypal_args' , 'change_paypal_args' );

function change_paypal_args( $paypal_args ) {
  global $wp;
  global $woocommerce;
  $order = wc_get_order( $order_id );

  $paypal_args['invoice'] = 'spi432';
  $paypal_args['txn_type'] = 'cart';
  $paypal_args['payment_date'] = $order->order_date;

    return $paypal_args;
}

I added txn_type and invoice as arguments to the link. But payment_date is not shown.

What may be the problem? Also, how can I display email of the customer?

If you enabled WP_DEBUG you would probably see that $order_id is undefined, and therefore $order is not an order object, so $order->order_date is likely a fatal error. Try passing the order as the second parameter instead.

add_filter( 'woocommerce_paypal_args' , 'so_42424283_change_paypal_args', 10, 2 );

function so_42424283_change_paypal_args( $paypal_args, $order ) {

    $paypal_args['invoice'] = 'spi432';
    $paypal_args['txn_type'] = 'cart';

    // WC 2.6+
    $paypal_args['payment_date'] = $order->order_date;

    // WC 2.7
    //$paypal_args['payment_date'] = $order->get_date_created();

    return $paypal_args;
}