Stripe + SilverStripe Omnipay

PaymentExtension.php

<?php

class PaymentExtension extends DataExtension
{
    private static $db = [
        'StripePaymentIntentReference' => 'Varchar(255)'
    ];
}

PurchaseServiceExtension.php

<?php

class PurchaseServiceExtension extends Extension
{
    public function onBeforeCompletePurchase(array &$data = [])
    {
        // Hack to get the payment, as silverstripe-omnipay doesn't currently
        // provide a getPayment() method in PaymentService
        $reflectionProperty = new ReflectionProperty(get_class($this->owner), 'payment');
        $reflectionProperty->setAccessible(true);

        $payment = $reflectionProperty->getValue($this->owner);
        if ($payment->StripePaymentIntentReference) {
            // Pass the Payment Intent reference with the transaction data to Stripe
            $data['paymentIntentReference'] = $payment->StripePaymentIntentReference;
        }
    }
}

YourPaymentController.php

<?php

// Ensure $data includes the token created by Stripe Elements JS
$data['paymentMethod'] = $stripeToken;

// Initialise the payment process
$response = ServiceFactory::create()
    ->getService($payment, ServiceFactory::INTENT_PURCHASE)
    ->initiate($data);

if ($response->isRedirect()) {
    $omnipayResponse = $response->getOmnipayResponse();
    if ($omnipayResponse instanceof Omnipay\Stripe\Message\PaymentIntents\Response) {
        // Store the Payment Intent reference for later...
        $payment->StripePaymentIntentReference = $omnipayResponse->getPaymentIntentReference();
        $payment->write();
    }
    return $response->redirectOrRespond();
}

z_example.js

/**
 * Payments
 */
(() => {
  const cardContainer = document.getElementById('card-element');
  if (!cardContainer) {
    return;
  }

  const init = () => {
    const stripe = Stripe(window.stripePublishableKey, {
      apiVersion: '2020-08-27'
    });
    const elements = stripe.elements();

    const cardElement = elements.create('card', {
      iconStyle: 'solid',
      style: {
        base: {
          color: "#000000",
          fontFamily: '"Montserrat", sans-serif',
          fontSmoothing: "antialiased",
          fontSize: "16px",
          "::placeholder": {
            color: "#707070"
          }
        },
        invalid: {
          color: "#840000",
          iconColor: "#840000"
        }
      }
    });
    cardElement.mount(cardContainer);

    const container = document.querySelector('.booking');
    const submitButton = document.querySelector('button.js-pay');
    const form = submitButton.form;
    const errorElement = document.getElementById('card-errors');

    submitButton.addEventListener('click', (event) => {
      event.preventDefault();
      errorElement.setAttribute('style', 'display: none;');

      stripe.createPaymentMethod({
        type: 'card',
        card: cardElement
      }).then((result) => {

        if (result.error) {
          // Inform the user if there was an error
          errorElement.textContent = result.error.message;
          errorElement.setAttribute('style', 'display: block;');
        } else {
          // Submit form
          form.querySelector('input[name="StripeToken"]').value = result.paymentMethod.id;
          form.submit();
        }
      });
    });
  };
  window.addEventListener('DOMContentLoaded', init);
})();

the line:

if ($omnipayResponse instanceof Omnipay\Stripe\Message\PaymentIntents\Response) {

needs to be replaced with:

if (is_a($omnipayResponse, 'Omnipay\Stripe\Message\PaymentIntents\Response')) {

on ss4 for some reason. the instanceof doesn't work for some reason.