Append the primary registrant's email address to the end of the Stripe order description. Event Espresso 4.

stripe_desc_email.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter(
    'FHEE__EEG_Stripe_Onsite__doDirectPaymentWithPaymentIntents__payment_intent_data',
    'stripe_elements_cf_order_desc_email',
    10,
    4
);
function stripe_elements_cf_order_desc_email(
    $stripe_data,
    $payment,
    $transaction,
    $billing_info
) {
  if ($transaction instanceof EEI_Transaction) {
    $primary_registrant = $transaction->primary_registration();
    if ($primary_registrant instanceof EE_Registration) {
      $attendee = $primary_registrant->attendee();
      if ($attendee instanceof EE_Attendee) {
        $stripe_data['description'] .= ' ' . $attendee->email();
      }
    }
  }
  return $stripe_data;
}