Amazon Pay has mismatched taxes when taxes are based on billing address

[daledupreez]

Description

When a store has taxes enabled and being computed based on billing address and Amazon Pay is enabled, we are hitting a situation where we only receive the shopper's billing address after they have confirmed the payment amount, but that may result in a different tax amount being applied during checkout as a result of the billing address being set. When the amount changes like this, the Amazon Pay payment is declined.

In the initial report, the symptom was the creation of a payment intent that didn't include any taxes for the express checkout, but that was one manifestation driven by the specific tax configurations involved.

Steps to reproduce

The issue here is partially driven by your store configuration, specifically around taxes for shopper locations and your billing address in Amazon Pay (which will drive which tax location you're in), as well as whether we have a billing address for the current user, either in the current session, or in the customer database. To get the best reproduction, it's best to test from a completely new, logged-out session, as we want to have no billing details set up for the user.

1. Enable the Amazon Pay feature flag - wp option update _wcstripe_feature_amazon_pay 'yes'
2. Configure your store to include taxes for purchases.
3. Configure the tax to be driven by the customer billing address.
   • Note: this step was missing in my initial report.
4. Enable Amazon Pay as an express checkout method, and ensure that it is displayed on the product page
5. Ensure you have a taxable product in your store.
6. Go to the product page for your product while in a new, logged-out browser session.
7. Click on the Amazon Pay express checkout button.
8. Log into the Amazon Pay sandbox or create a new sandbox account.
9. Pick or create a supported shipping address.
10. Pick a valid card.
11. Note the total includes taxes for the item.
12. Complete the checkout.
13. Note that the payment fails. If you then review the logs, you can see that we're creating a payment intent for the item total plus shipping, but we are sending a different amount for the taxes.

Expected behavior

The payment should include the correct taxes, and it should be processed successfully.

Additional context

[wjrosa]

As we discussed earlier, Amazon Pay is working for me with the setup above. I would appreciate if you can share your specific setup information so I can try to reproduce the issue again when you have the chance.

[daledupreez]

Thanks for testing, @wjrosa! As we worked out, we also need taxes to be based on the customer billing address. I've updated the steps above to reflect that by adding what is now step 3.

[wjrosa]

I did some comparison between the data we send to Stripe when taxes are based on both the shipping and billing address, and they are almost the same (except for the specific shipping taxes). The Stripe error message does not contain any relevant information (we get a generic_error code). So I asked for their help in p1761401219511239-slack-C9976E5MJ

[daledupreez]

I did some additional testing and debugging today, and I now have a much more robust sense of what is going wrong. The core sequence/behaviour is as follows:

• When we send the shopper to Amazon, we may or may not have a billing address for them depending on whether they are logged in or not, so our tax calculations could easily be wrong.
  • Issues here could include: the default shopper location is used to start, or we have a saved address for the shopper that doesn't match their Amazon billing address, or we don't have a billing address and are using the shipping address to compute taxes.
• The shopper then approves a specific amount based on that initial tax calculation, though the amount may be adjusted based on the selected shipping method and/or address.
• We get notified of the express checkout confirmation (via the confirm event), which includes the shopper's billing address, and we then create a confirmation token in the client.
• We then initiate checkout using the updated/current customer details, and that may lead to taxes being adjusted due to the updated billing address. However, if the customer billing address update resulted in tax changes, our attempt to process the payment gets declined.

For context, I was testing with a California-based store, a shopper with their Amazon billing address in Wisconsin and Amazon shipping addresses in Illinois and Massachusetts. I had the taxes set up as follows to make sure that switching the address would result in different tax amounts:

• US-CA - 7.25%
• US-IL - 7.0%
• US-MA - 6.25%
• US-WI - 5.0%

[daledupreez]

I also explored forcing an update to the express checkout amount after we get the billing address in the confirm event, which was triggering a request to update the payment amount, but that was still resulting in the payment being declined. The code for that is up in #4768.