Cancel a subscription. Use when a customer churns or downgrades. Supports immediate or end-of-period cancellation with optional proration.
"sub_x1y2z3a4b5c6d7e8f9g0""IMMEDIATE" - Cancel immediately"END_OF_PERIOD" - Cancel at the end of the current billing period"NONE" - No proration"CREATE_PRORATIONS" - Generate proration credits/charges"SKIP" - Do not generate invoice"GENERATE" - Generate final invoice"Customer requested cancellation""CANCELLED", "ACTIVE" (if cancelling at period end)