Update Quote/Item/Processor to allow custom price zero with free payment method

[ianitsky]

Fix free payment method for products with custom price zero

Description (*)

I use custom prices to change the product price dynamically but if I set the custom price of a product as zero (for free shopping), the free payment method becomes unavailable.

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes Custom Price of 0 gets reset to original price on reorder. #36970

Manual testing scenarios (*)

1. Create a product with any price
2. Enable free payment method.
3. Use graphql to add a product in the cart and set the custom price to zero.
4. Place order

Questions or comments

Contribution checklist (*)

• [*] Pull request has a meaningful description of its purpose
• [*] All commits are accompanied by meaningful commit messages
• [*] All new or changed code is covered with unit/integration tests (if applicable)
• [*] README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• [*] All automated tests passed successfully (all builds are green)

[m2-assistant]

Hi @ianitsky. Thank you for your contribution!
Here are some useful tips on how you can test your changes using Magento test environment.

Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento give me test instance - deploy test instance based on PR changes
• @magento give me 2.4-develop instance - deploy vanilla Magento instance

❗ Automated tests can be triggered manually with an appropriate comment:

• @magento run all tests - run or re-run all required tests against the PR changes
• @magento run <test-build(s)> - run or re-run specific test build(s)
  For example: @magento run Unit Tests

<test-build(s)> is a comma-separated list of build names.

Allowed build names are:

1. Database Compare
2. Functional Tests CE
3. Functional Tests EE
4. Functional Tests B2B
5. Integration Tests
6. Magento Health Index
7. Sample Data Tests CE
8. Sample Data Tests EE
9. Sample Data Tests B2B
10. Static Tests
11. Unit Tests
12. WebAPI Tests
13. Semantic Version Checker

You can find more information about the builds here
ℹ️ Run only required test builds during development. Run all test builds before sending your pull request for review.

---

For more details, review the Code Contributions documentation.
Join Magento Community Engineering Slack and ask your questions in #github channel.