Restrict Affirm and Klarna to domestic and cross EEA border transactions

[james-allan]

Fixes #3096

Changes proposed in this Pull Request:

Customers paying with Klarna, Affirm and Afterpay/Clearpay have specific cross border restrictions they must adhere to. ie Customers can only purchase from the same country the Stripe account is located. eg AU to AU. The one exception to this is Klarna where customers and merchants located in the European Economic Area (EEA) + the UK and Switzerland can also transact. eg a customer in the UK can transact with a merchant in Germany.

This is documented in Stripe docs:

• https://docs.stripe.com/payments/klarna
• https://docs.stripe.com/payments/afterpay-clearpay
• https://docs.stripe.com/payments/affirm

This PR implements this restriction by ensuring that the payment method is only enabled on checkout if the transaction is considered domestic using their respective ruleset.

Testing instructions

Strictly Domestic - eg US.

1. Add Stripe API credentials for a country that is eligible for Klarna eg US.
2. Make sure to set your store currency to the matching currency.
3. Enable Klarna as a payment method.
4. Add a simple product to your cart.
5. Go to the checkout.
6. Change your billing country to a country that doesn't match your Stripe Account's base country. eg if your account is in the US, set it AU or something else.
7. On this branch Klarna should not be available.
8. Change your billing country to match the stripe accounts country.
9. It should now be shown.

EEA

1. Create a EU based Stripe Account
2. Add Stripe API credentials for a country that is eligible for Klarna and located in the EU eg France.
3. Make sure to set your store currency to the matching currency eg Euro.
4. Enable Klarna as a payment method.
5. Add a simple product to your cart.
6. Go to the checkout.
7. Change your billing country to a country any country in the EU that doesn't match your Stripe Account's base country. eg if your account is in the France, set it Germany or something else.
8. Klarna should be available.
9. Change your billing country to match the Stripe account's country.
10. Klarna should be available.
11. Change your billing country to the US or AU etc.
12. Klarna shouldn't be available.

---

• 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)

[james-allan]

Placing this one on-hold as I think there's a better way to approach this and it will fix it for the Block checkout too.

[james-allan]

@a-danae do you mind reviewing this one. I've tweaked your code that you introduced in #3040 so wanted to double check this is inline with your understanding of the code you introduced.

[james-allan]

This function was only used to populate the JS data with a list of eligible countries. Given I've changed it's purpose to return billing countries, I changed the name. Because this function is unreleased, there are no backwards compatibility concerns here.

[a-danae]

Thanks for tackling this, @james-allan ! Things look good to me 👍

I tested the following scenarios:

US Stripe account
✅ Domestic country - US
✅ Non-domestic, Klarna-supported country - AU
✅ Non-supported country - MX

France Stripe account
✅ Domestic country - FR
✅ Country from within the EEA - ES
✅ Non-domestic, Klarna-supported country - AU
✅ Non-supported country - MX

UK Stripe account
✅ Domestic country - UK
✅ Non-domestic, Klarna-supported country - AU
✅ Non-supported country - MX

🟡 Cross-border transactions where the currency of the Stripe account and the customer billing country don't match are failing. We could address that separately - #3118

This PR also fixed a console error we were getting when changing the billing country from a non-supported to a supported one 🎉

Glad to see this merged. Thanks again!

[a-danae]

Merging this PR to create the test package!