In-app purchase seam: run the platform store's native purchase flow (Android Google Play; desktop/web install no handler — they bill via Stripe, so this is a safe no-op there). Async like the upload seam: the shell launches the store sheet for productId/planId (attaching accountId, an opaque per-account id the store hashes for fraud signals) and delivers a purchase token back. Only a string crosses the boundary — the server verifies the token, so no billing logic lives in the core.