Fix subscription processing with mandates

[wjrosa]

Fixes #3230

Changes proposed in this Pull Request:

This PR fixes the processing of subscriptions when using mandates (i.e., when using Indian credit cards). This is done by forcing the amount_type parameter to maximum and the interval parameter to sporadic in all cases. More information on this here.

Testing instructions

• Instructions based on the testing steps of the original issue
• Checkout to this branch on your local environment (fix/fix-subscription-processing-with-mandates)
• Set your store and Stripe account currency to Indian Rupee
• Install and enable the subscriptions extension, and enable switching on wp-admin/admin.php?page=wc-settings&tab=subscriptions
• Create a variable subscription with two variations with prices ₹1,000/month and ₹2,000/month.
• First, buy ₹1,000/month subscription on, say, the 1st of the month. This will set up an autopay mandate of ₹1,000.
• Now upgrade to the ₹2,000/month subscription in the middle of the month *. The switch will happen successfully
• On the renewal day*, the payment should succeed
• On the develop branch it should fail

* You can simulate both by editing the _schedule_start and _schedule_next_payment metas. Subtracting 15 days from each

---

• 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)
• Added the needs docs label (or does not apply)
• Included this PR in the Release Thread scope (or does not apply)

[james-allan]

@wjrosa I've been trying to replicate this using a different set of instructions that might make it easier to test.

1. Connect to an India based Stripe account.
2. Set store to Indian Rupee
3. Purchase a subscription using an India based testing card. eg 4000003560000008 from here.
4. Purchase a subscription.
5. Edit the subscription from WooCommerce > Subscriptions.
6. Edit the line items and increase the number of items. eg x 2. This will change the total.
7. From the actions drop down attempt to process a renewal.

8. The order will fail and looking in the Stripe logs it will fail with this error:

9. On this branch, if I repeat the process, the mandate is created with a sporadic and maximum amount type, but my subscriptions are still failing with an invalid mandate.

I'm not really sure what's happening there but I wanted to comment my findings before I need to move on to test other things.

I also wanted to note that Stripe's API docs suggest that the value in the amount param when maximum is type is the maximum amount we can charge using that mandate. That might be the problem here but I haven't been able to confirm.

^{source: https://docs.stripe.com/api/payment_intents/create#create_payment_intent-payment_method_options-card-mandate_options}

[wjrosa]

Thanks for sharing your findings, James! I asked the Stripe team about this issue on p1723152195794419-slack-C9976E5MJ . I hope they can help us find a proper solution 🙏

[wjrosa]

Closing in favor of #3359