Write a test strategy for a small, consequential release.

Fictional case: a subscription service adds a self-service cancellation screen. Cancellation must stop the next renewal without deleting the account, retain access until the paid-through instant, and be safe to repeat. Billing timestamps are UTC. Existing customers have monthly or annual plans; some have a renewal already in progress. An existing cancellation API changes only to return the paid-through instant. QA has two working days; engineering owns automated component and API checks and can run them concurrently. A sanitized staging payment simulator can pause renewal before and after charging; no real card or customer data is permitted. Product owns entitlement policy, the billing engineer owns transaction correctness, and the release manager owns the release decision with security approval required for access controls. Existing login and account-isolation checks are mandatory. The release can remain behind a server-side flag; existing support-assisted cancellation remains available.