Improve ECE compatibility with One Page Checkout

[malithsen]

Fixes STRIPE-743

Changes proposed in this Pull Request:

This PR improves the compatibility between Stripe Express Checkout (ECE) buttons and the WooCommerce One Page Checkout (OPC) plugin.

Problem:
When using the OPC plugin with ECE configured to show only on "checkout pages":

1. ECE buttons don't appear on OPC pages because is_checkout() returns false (they're technically product pages)
2. When buttons did appear with both locations enabled, they showed in the product section instead of the checkout section.
3. ECE buttons displayed original product prices instead of discounted prices after applying coupon codes

Changes proposed:

• Detect OPC pages using is_wcopc_checkout() and treat them as checkout context rather than product context
• Skip product page restrictions for OPC pages when checkout buttons are enabled
• Force frontend to use cart totals (which include discounts) instead of product pricing on OPC pages when checkout buttons are enabled

Testing instructions

Prerequisites

• WooCommerce One Page Checkout plugin installed and active

Test Case 1: ECE shows on OPC with checkout location enabled

1. Go to WooCommerce > Settings > Payments > Stripe
2. Under "Express Checkout", set button locations to "Checkout" only (uncheck Product and Cart)
3. Create a product and mark it as One Page Checkout enabled (OPC meta field _wcopc = 'yes')
4. View the product page
5. Expected: Express Checkout buttons (Apple Pay, Google Pay, etc.) appear in the checkout section of the OPC
6. Expected: No ECE buttons appear in the add-to-cart section

Test Case 2: Correct pricing with discount codes on OPC

1. With the same setup as Test Case 1
2. On the OPC product page, apply a coupon code in the checkout section
3. Observe the ECE buttons
4. Open an ECE (eg: Google Pay), the total in the payment modal should have the discount applied

Optional:
Test for regressions with OPC disabled.

CleanShot.2025-11-03.at.12.11.26.mp4

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• Covered with tests (or have a good reason not to test in description ☝️)
• Tested on mobile (or does not apply)

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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)
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[daledupreez]

Functionally, this is working as described. I also tested the situation where express checkout is enabled in the product page and checkout, and confirmed that we're still showing only the express checkout buttons in the One Page Checkout section and not in the main product section. I also smoke tested loading an ECE modal from the button without One Page Checkout active, and that was clear, as were the related e2e tests -- there were some ACH failures, but those are being worked on in a dedicated PR.

However, I think there are some test-related improvements we can make, or at least discuss before we merge this PR.

[diegocurbelo]

The changes look good and work as described.

[daledupreez]

Thanks for the updates to the tests! 🚢