Epic: Migrate Stripe PRB to ECE

[bborman22]

The goal of this project is to migrate from Stripe's PaymentRequestButton to Stripe's ExpressCheckoutElement for express checkout buttons in WooPayments.

In doing this migration we aim to increase the number of express payment and wallet options available in WooPayments. We also will aim to address as many existing issues with PRB today as we can using the new tools available with ECE. The project will be considered complete once all express checkout options are using ECE and we've cleaned up the unused PRB code.

Reading Materials
Early exploration into ECE: pbIwZe-1QB-p2
Description of some of the problems we're facing with PRB: paJDYF-ckK-p2
Early Layout of Phases: #8501

Milestones

Milestone 1 - Render behind a feature flag and initial test payment in WC Blocks.

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1. Render the payment buttons using ECE behind a feature flag. #8772
   focus: checkout payments +1
   
2. Integrate ECE for Payment Processing in the Blocks Checkout Page #8773
   focus: checkout payments +1
   
3. Refactor codebase to decouple PRB utils dependencies #8825
   focus: checkout payments +1
   

Milestone 2 - Feature parity with PRBs.

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1. Integrate ECE for Payment Processing in the Blocks Cart Page #8867
   focus: checkout payments +1
   
2. Integrate ECE for Payment Processing in the product page #8868
   focus: checkout payments +1
   
3. Integrate ECE for Payment Processing in the Shortcode Cart/Checkout #8870
   focus: checkout payments +1
   
4. Integrate ECE Payment Processing for the Pay for Order Endpoint #8869
   focus: checkout payments +1
   
5. Ensure ECE Implementation Supports the Same Plugins and Product Types as PRBs #8871
   focus: checkout payments +1
   
6. Spike: Investigate if changes are needed to support Apple Pay in ECE #8873
   focus: checkout payments +1
   
7. Spike: investigate compatibility between Link and WooPay when Link is offered in ECE #8874
   focus: checkout payments +1
   
8. Spike: investigate if ECE payment methods can be registered individually #8872
   focus: checkout payments +1
   
9. Disable Stripe Link in the Express Checkout Element Before Migration #9035
   focus: checkout payments needs prioritisation +2
   
10. [ECE] Add support for WooCommerce Deposits. #9067
    focus: checkout payments +1
    
11. Fix payment method title for Express Checkout Element orders #9076
    focus: checkout payments type: bug +2
    

Milestone 3 - Design.

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1. Ensure Button Settings are applied in ECE and Obtain Design Approval #8951
   4 of 4
   focus: checkout payments needs design +2
   
2. Update the settings preview to use ECE #9007
   focus: checkout payments +1
   
3. [ECE] Fix incorrect button widths #9005
   focus: checkout payments priority: high type: bug +3
   
4. [ECE] Button max-height is 55px due to constraints in Stripe's API #9006
   focus: checkout payments priority: medium type: bug +3
   
5. Register ECE Payment Methods Individually in the WC Blocks API. #9045
   focus: checkout payments +1
   
6. [ECE] Finalize button presentation and design #9066
   4 of 4
   component: payment request button focus: checkout payments impact: high needs design priority: high type: enhancement +6

Milestone 4 - Telemetry, security, performance, reliability.

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1. Ensure the Telemetry events for PRBs are Implemented into ECE #8948
   focus: checkout payments +1
   
2. Exploratory Testing Session for ECE.
3. Implement error logging in ECE #9049
   focus: checkout payments +1
   
4. [ECE] Disable ECE for non-shipping products if tax is based on billing address #9080
   focus: checkout payments +1
   

Milestone 5 - Launch.

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1. Set ECE as Default for Express Checkout Buttons #8949
   focus: checkout payments +1
   

Milestone 6 - Post-launch, PRBs removal.

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1. Remove ECE feature flag and clean-up PRB code #8950
   3 of 3
   focus: checkout payments +1
   

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Future Work - Phase 2

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1. Refactor ECE to Reduce or Remove jQuery Dependency #9074
   focus: checkout payments +1
2. Ensure ECE Compatibility with the Cart Tokenization Project #8947
   focus: checkout payments size: x-small status: blocked +3
3. Enable Stripe Link in the Express Checkout Element #9034
   focus: checkout payments needs design needs feedback needs prioritisation +4
4. Order data missing payment information in Pay for Order flow with ECE/PRB #9048
   focus: checkout payments priority: medium type: enhancement +3
5. Spike: Inconsistent tax behavior when using Express Checkout for virtual products #8373
   focus: checkout payments priority: high status: blocked type: bug +4
6. Spike: Investigate opportunities for PHP unit tests in the ECE implementation. #9026
   focus: checkout payments +1