Integrating ECE to shortcode checkout

[wjrosa]

Fixes #3404
Doc: https://docs.stripe.com/elements/express-checkout-element

Changes proposed in this Pull Request:

This PR introduces the new Stripe Express Checkout Element to our shortcode checkout, replacing the current PRB version.

This integration is heavily based on WCPay's, with just some adjustments to adapt the code to our needs and standards.

Testing instructions

• Apply this PR to your test environment (add/ece-for-shortcode-checkout)

• Make sure it is publicly accessible. You can use ngrok for this (or a JN website)

• Enable the ECE flag. You can hardcode is_stripe_ece_enabled to true

• As a merchant, connect your Stripe account and create a shortcode checkout page

• As a customer, add any product to your cart

• Go to the shortcode checkout page

• Confirm that the new ECE buttons are loaded:
  

• Confirm that you can place an order successfully

---

• Covered with tests (or have a good reason not to test in description ☝️)
• Added changelog entry in both changelog.txt and readme.txt (or does not apply)
• Tested on mobile (or does not apply)

Post merge

• Added testing instructions to the Release Testing Instructions wiki page (or does not apply)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

cc @Mayisha

[mattallan]

Thanks for kicking these changes off Wesley, I've just taken a quick code review and left some notes.

I will base my branch off this PR to start looking at implementing ECE on the product page

[wjrosa]

Thanks, @Mayisha ! Can you and @james-allan test this again now that I have rebased it with your newly created PR?

[Mayisha]

QQ - Is the wording of this message and the comment above intentionally changed from express checkout to Payment requests?

[wjrosa]

I think so, to make sense with the class name. This specific line was changed by @mattallan

[wjrosa]

@Mayisha, I have fixed the issue you reported on Slack. Some parameters were missing from the fake cart data provided. Now, it is working fine for me (I hope it fixes it for you folks, too).

[james-allan]

This looks good to me. I tested a couple of different general scenarios.

• Basic purchases as a logged in user and logged out (as a guest).
• Subscription purchases.
• Subscription renewals.
• Free trial subscriptions. (Not available - see is_invalid_subscription_product())
• Errors - I'm not sure if there's good way to test failures, however, I forced errors by invaliding the payment request.

• I changed the checkout button settings in the plugin and confirmed they are reflected on the checkout

• 🔴 I checkouts with a coupon applied. This failed and the checkout remained blocked.

• 🟢 I tested various shipping setups and confirmed that the order total in the model and the Stripe Dashboard was correct.
  • I chose a shipping method, changed it via the Google Pay pop up. See example video below.

https://github.com/user-attachments/assets/f09ed8fb-2bfa-4545-98e3-48c258a7e35c\

I checkouts with a coupon applied. This failed and the checkout remained blocked.

I'm happy if we want to handle this separately given the large size of this PR and the other work depending on this PR being merged.

@Mayisha I'm not sure if there are other specific tests you had you want us to check as part of this feature. Let me know if there are specific flows we should check etc. :)

[Mayisha]

Thanks @wjrosa for the implementation. I have tested with Google Pay and everything is working on the shortcode checkout page.

✅ Purchasing simple product, digital product, simple subscription work.
✅ Checkout with different shipping methods (free, flat rate) works. Tested by changing the shipping method both from checkout page and Google Pay modal.
✅ I can not reproduce the issue related to shipping address anymore.

Thanks @james-allan for testing other scenarios. I have created separate issues for the coupon and payment method title errors.

• [ECE] Fix payment method title for orders paid with ECE button  #3469
• [ECE] Payment processing with coupon and discount #3471