CHW-4401 BillingPlanInvalid
plan_id is not one of the allowed plan slugs; supply a current plan from /api/billing/plans
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Default message: plan_id is not one of the allowed plan slugs; supply a current plan from /api/billing/plans
HTTP status:400(client)
Problem. /api/billing/checkout rejected the supplied plan_id.
Cause. The plan slug is not one of the allowed values seeded in
pricing_plans / validPlanIDs. Fix. Call /api/billing/plans to
enumerate current plans and retry with a returned slug. HTTP 400.