fix(ras-acc): respect cart coupons on nyp submit

[chickenn00dle]

All Submissions:

• Have you followed the Newspack Contributing guideline?
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Changes proposed in this Pull Request:

Closes https://app.asana.com/0/1207817176293825/1208344897866336/f

This PR fixes an issue where the checkout button nyp input would remove any applied coupons on submit.

How to test the changes in this Pull Request:

1. Set up a checkout button block for a product that has both coupons AND nyp enabled
2. As a reader trigger the checkout modal via the checkout button and go through checkout until you get to the payment method section
3. Input a valid coupon and submit. Verify the order review table and submit button update with the correct values
4. Now input a valid NYP amount and submit. On epic/ras-acc the coupon will be removed. On this branch, it should not and everything should update correctly.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

Good find! This fixes the issue! 🙌