Add allow_promotion_codes: true to the checkout.sessions.create call in
POST /checkout. This is what renders the "Add promotion code" field on Stripe's
hosted checkout page; for API-created sessions there is no Dashboard equivalent
(that toggle only exists for Payment Links, which we don't use), so a comment
warns against removing it as "redundant". The billingPortal branch is untouched
— portal sessions handle discounts separately.
Testing: no Stripe-SDK test/mock existed (the billing-*.test.js files cover the
#146 usage-metering path, not Stripe). Added stripe-checkout.test.js using the
repo's in-process router-mount convention with a minimal `stripe` stub injected
via require.cache, asserting the checkout payload carries
allow_promotion_codes:true (and still builds a subscription session for the
requested price). Full suite 557/557.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
