How to add a subscription add-on with immediate proration WITHOUT re-billing the base plan

Hi,

We're using PUT /subscription/{id}/upgrade-downgrade to let customers add an add-on to their existing subscription at any point in their billing cycle, without changing their plan.

Payload we send (same plan_id as the customer's current plan — we are not changing plans, only adding an add-on):
{
  "plan_id": "23222d219q21231234252b7a6",
  "customer_id": "66272321312323ce9ed2a060",
  "payment_term": "net0",
  "activated_at_val": "immediately",
  "payment_mode": "credit_card",
  "addons": [
    { "id": "234236215423421231223474", "quantity": "1" }
  ]
}

What we observed: the customer's card was charged (attempted) for $200 MXN, which is the full monthly plan price ($150) plus the add-on price ($50) — not just the add-on. Since plan_id didn't change, we expected only the add-on's prorated cost to be charged immediately, with the base plan continuing on its normal billing cycle.

Our question: is there a documented way to add an add-on to an existing subscription with activated_at_val: "immediately" so that only the add-on is charged now (prorated for the remaining period), while the base plan itself is left untouched and simply bills together with everything else at the next renewal — instead of the entire subscription (plan + add-on) being re-invoiced immediately?

Is plan_id required even when unchanged? Is there a separate endpoint or parameter set for "add-on only" changes that avoids re-prorating the base plan?

Thanks in advance.